The U.S. military says its airstrikes killed 12 Islamic State operatives in Syria.

Coordinating with “our Russian allies.” Mr. al-Assad said, he then headed to the coastal city of Latakia, near where Russia operates a military base.

The rebel leader, Ahmed al-Shara, meanwhile, gave his own public statements in an interview with journalists on Monday. He urged the United States and other countries to remove their terrorism label for Hayat Tahrir al-Sham, his militant group, and for the lifting of sanctions previously placed on the Assad regime. He emphasized that they had been imposed on “the executioner”— meaning Mr. al-Assad, who was now gone — and that Syria needed to prioritize building a state and creating public institutions that served all residents.

Here’s what else to know:

• Assad statement: Tass, the Russian state news agency, said Mr. al-Assad’s statement had been issued from Moscow. The statement later appeared to have been removed from one of the social media accounts of the former Syrian presidency.

• Syria diplomacy: As the world reckons with the rise to power of Hayat Tahrir al-Sham, a conservative Islamist group long designated a terrorist organization, Mr. al-Shara met on Sunday with the U.N. special envoy to Syria, Geir O. Pedersen. The new government was also expected to meet with leaders from across Europe and the Middle East. Many international powers have said they want to see a stable, unified Syria with an inclusive government that respects the rights of Syria’s minorities, and they now have the leverage to coax the new leadership toward that notion.

• Russian moves: As the Kremlin acknowledged that the fate of its critical bases in Syria was uncertain, video reporters from The New York Times observed a convoy of Russian military vehicles moving from Tartus to Latakia, two cities in western Syria where it has military bases.

• Israeli strikes: Israel carried out another heavy wave of airstrikes overnight on Syria’s coastal region, a war monitor said on Monday, as the Israeli military continued to pound Syria in a bid to destroy the country’s military assets after the rebel alliance seized power. Israel has said it aims to keep military equipment away from extremists.

“CENTCOM, working with allies and partners in the region, will not allow ISIS to reconstitute and take advantage of the current situation in Syria,” Gen. Michael Erik Kurilla, Centcom’s commander, said in the statement.

Just over a week ago, the United States conducted one of its largest strikes in Syria against ISIS in months, saying that it hit “over 75 targets.” Earlier this year, the Pentagon warned that the group’s attacks in Iraq and Syria were on track to double.

The latest U.S. strikes came on the same day that the Kurdish-led administration governing much of northern Syria pleaded for unity and for a countrywide cease-fire. Kurdish forces backed by the United States have come under attack from Turkish-backed rebels on multiple fronts.

Those rebels, supported by Turkish air support and drones, are fighting to take territory from the Kurdish administration. The top commander of the the main Kurdish militia, the Syrian Democratic Forces, said the assaults had forced fighters to be diverted from defending the prisons that house people accused of being ISIS members.

Amid the fighting in northern Syria, other parts of the country have been pummeled by an Israeli aerial campaign. Since the fall of Bashar al-Assad and his government, Israel has sought to destroy Syria’s military assets.

Mr. Putin has yet to say anything in public about the collapse of his close ally, Bashar al-Assad, in Syria more than a week ago, even as Russia struggles to salvage what influence it can in the Middle East. The silence underscores the uncertainty surrounding the future of Russia’s military bases in Syria — and the overwhelming priority for the Kremlin that the war in Ukraine has become.

Mr. al-Assad’s fall is a painful topic right now in Moscow, said Anton Mardasov, a Moscow-based military analyst focusing on the Middle East. “It’s better not to say anything.”

Things were very different just a year ago, when Sergei K. Shoigu, then the defense minister, boasted at the same annual meeting that Russian troops remained deployed both in Syria and Nagorno-Karabakh, the Armenian-populated enclave that Azerbaijan recaptured from Armenia last year.

“Russian groups of forces remain the backbone and the main guarantee of peace in Syria and Karabakh,” Mr. Shoigu said at the time.

Russian peacekeeping forces pulled out of Nagorno-Karabakh in May, a sign of Russia’s loss of influence in the Caucasus region, which had been part of the Soviet Union.

Now Mr. al-Assad’s ouster could become an even greater setback to Mr. Putin’s efforts to revive Russia as a world power. Mr. Mardasov said a best-case scenario for Russia could be a scaled-down military presence at the Hmeimim air base and the Tartus naval base in the Mediterranean. That could allow Moscow to keep a refueling and staging point for limited military activity there and in Africa.

But that scenario wouldn’t fulfill Mr. Putin’s earlier, broader ambitions of projecting might on the doorstep of NATO. Russian nuclear-capable bombers flew training missions from Syria in 2021, a signal that Mr. Putin saw his military’s presence in the country as a bulwark in his global conflict with the West.

Now, Mr. Mardasov said, the security situation in Syria is likely to remain so tenuous that Russia wouldn’t be able to station nuclear-capable weaponry there even if it struck an agreement to hold on to its bases.

“Such outposts to threaten NATO’s southern flank are already 100 percent lost,” Mr. Mardasov said. “Even if they manage to keep a presence, it will be symbolic.”

To Mr. Putin, the outcome of the war in Ukraine has now become the biggest factor in Russia’s future security. And in that war, he believes he is winning, both on the battlefield and in his standoff with the West, as politicians skeptical of supporting Ukraine, led by President-elect Donald J. Trump, increasingly come to power.

The government in Germany, one of Ukraine’s largest supporters, collapsed on Monday, and the cost of the war is one of the issues likely to dominate the upcoming election campaign. On Thursday, the leaders of the European Union’s 27 member countries are scheduled to meet in Brussels to discuss, among other things, a path forward in Ukraine.

In Syria, Russia has apparently been scaling down its presence in recent days, with convoys of troops that had been stationed across the country seen pulling back to Russia’s Hmeimim and Tartus bases. Satellite images last week showed Russian military equipment being packed up for loading onto transport planes.

Russian officials have sought to engage with Hayat Tahrir al-Sham, the conservative Islamist group that led the rebel offensive that toppled Mr. al-Assad. Dmitri S. Peskov, the Kremlin’s spokesman, told reporters Monday that here had been “no final decisions” on the future of Russia’s military presence in Syria.

“We are in contact with representatives of the powers that currently control the situation in the country,” Mr. Peskov said. “All this will be determined in the course of the dialogue.”

Just hours before Mr. al-Assad fell on Dec. 8, Russia’s top diplomat, Sergey V. Lavrov, was still describing the Syrian rebels as “terrorists.” In one sign of Russian hopes for rapprochement, one of the country’s most powerful men, the Chechen strongman Ramzan Kadyrov, said that Hayat Tahrir al-Sham should be taken off Russia’s terror list.

Mr. Kadyrov, who rules the majority-Muslim Russian republic of Chechnya, also said that Chechen authorities were willing to go on joint patrols “with the Syrian law enforcement agencies.”

It’s not clear whether Moscow’s diplomacy will be enough, however, to allow Russia to keep a military presence. Some European Union officials said Monday that they would seek to make Russia’s exit from Syria a condition of lifting sanctions on Syria.

“Many foreign ministers emphasized that it should be a condition for the new leadership to eliminate Russian influence,” Kaja Kallas, the European Union’s foreign policy chief, told reporters in Brussels.

But Mr. Putin made clear once more on Monday that Syria had become a secondary concern. In his speech to the military leadership, he claimed his troops “hold the strategic initiative” along the entire front line in Ukraine and that the flow of Russians volunteering to fight “is not stopping.”

It was fresh evidence that Mr. Putin believes he can outlast Ukraine on the battlefield, even as Mr. Trump promises to negotiate a peace deal to end the war. Mr. Putin’s defense minister, Andrei R. Belousov, said in Monday’s meeting that “ensuring victory” in the Ukraine war was the military’s top priority, but he said nothing about its plans or aims in the Middle East.

Russian state television has sought to fill the silence with claims that Russia fulfilled its mission in Syria and that any instability there is now the West’s fault. On the marquee weekly news show on the Rossiya channel on Sunday, the host, Dmitri Kiselyov, said that Russia had made contact with “the leaders of the armed opposition” and that both sides were showing “mutual restraint.”

“Russia did all it could to leave calm and stability in Syria,” Mr. Kiselyov said. “Russia has now taken a pragmatic position.”

Oleg Matsnev contributed reporting from Berlin, and Alina Lobzina from London.

Israel has framed its advance onto Syrian territory as a defensive measure, to ensure militants do not take up positions on its border. On Monday, al-Shara countered that Israel no longer needed to hold that land to protect itself because the toppling of the Assad government had removed the threat from Hezbollah and other Iranian-backed militias.

Mr. Tice’s mother, Debra Tice, said in the letter, dated Dec. 14 and addressed to Mr. Netanyahu, that the prison was under a Syrian military museum in the mountainous Mount Qasioun area and had a tunnel that was connected to a neighborhood and a government palace.

“We are aware that your military has an active campaign in the area, preventing rescuers from approaching and accessing the prison facility,” Ms. Tice wrote in the letter, a copy of which was obtained by The New York Times.

“We have no way of knowing if the prisoners there have food and water. We urgently request you pause strikes on this area and deploy Israeli assets to search for Austin Tice and other prisoners. Time is of the essence.”

Mr. Netanyahu’s office did not immediately respond to a request for comment.

The Israeli military has been bombing weapons depots and air defenses in Syria, according to the Syrian Observatory for Human Rights, a Britain-based organization that tracks the conflict in Syria. Israel has said it wants to keep military equipment away from extremists.

Mr. Tice was abducted in 2012 outside Damascus as the country descended into civil war. The United States has said it believes he was being held by the government of Bashar al-Assad. The Assad regime had long maintained that it was not holding Mr. Tice and had no information about him.

Mr. Tice’s family and the United States government have stepped up efforts to locate him since Dec. 8, when rebel groups seized Damascus after overrunning several other major cities, and Mr. al-Assad fled to Russia.

As for terrorism designations of Al-Shara himself, as imposed by the United States and other countries, he said, “That is not very important to me.”

Earlier Monday, Dmitri S. Peskov, the Kremlin’s spokesman, said there had been “no final decisions” on the future of Russia’s two military bases in Syria. He added that Russia was “in contact” with the new Syrian authorities.

Mr. al-Assad said that he had not planned to leave the country, and said he did not “consider stepping down or seeking refuge” as the rebels advanced. He said he had remained in Damascus, the capital, “carrying out my duties” until early on Dec. 8, when, he said, rebels began to infiltrate the city. At that point, he moved “in coordination with our Russian allies” to the Syrian coastal city of Latakia, and later arrived at Russia’s Hmeimim air base nearby, he said.

“As the field situation in the area continued to deteriorate, the Russian military base itself came under intensified attack by drone strikes. With no viable means of leaving the base, Moscow requested that the base’s command arrange an immediate evacuation to Russia on the evening of Sunday Dec. 8,” Mr. al-Assad said in the statement, which was published in English. This account of the episode could not be independently confirmed.

The Kremlin has said that Mr. al-Assad made the “personal decision” to leave office and that President Vladimir V. Putin had offered exile to him and his family. Dmitri S. Peskov, the Kremlin spokesman, has said Moscow will not disclose details of Mr. al-Assad’s location in Russia, which has been a staunch ally of Mr. al-Assad.

The Assad family ruled Syria with unrelenting force for more than 60 years. Since the rebel coalition overthrew Mr. al-Assad’s government over a week ago, Syrians have begun to reckon with the network of prisons, police stations and torture chambers that were central to his family’s brutal rule, and the abuses of the past 13 years, after the failed rebel uprising and subsequent civil war. Death toll estimates from the conflict are as high as 620,000, in a country with a prewar population of 22 million.

Mr. al-Assad’s arrival in Russia has effectively put him out of the reach of international justice, human rights advocates have said.

In the statement, Mr. Assad offered a defense of his record in office, saying he had “refused to barter the salvation of his nation for personal gain.” Mr. al-Assad, who at times in the one-page statement referred to himself in the third person, also did not mention any plans for his future, and said that his bond with Syria and its people remained “unshaken.”

“When the state falls into the hands of terrorism and the ability to make a meaningful contribution is lost any position becomes void of purpose,” he said, in an apparent attempt to provide a larger explanation for his departure. There was no immediate public response to the statement from Syria’s new transitional government.

Speaking to reporters on his arrival in the Syrian capital, Damascus, Mr. Pedersen said many challenges lay ahead for Syria and called for increased aid to assist with the country’s humanitarian crisis.

Kaja Kallas, the European Union’s foreign policy chief, told reporters on Monday that she had sent the “European top diplomat in Syria” to meet with the new government in Damascus. The European Union is the biggest donor of humanitarian aid to Syria through U.N. agencies, making the relationship with Brussels a crucial one.

France’s foreign ministry said on Sunday that a team of diplomats would travel to Syria on Tuesday. And Turkey and Qatar, which were in contact with the rebels well before the surprise offensive that rocketed them from obscurity in Syria’s northwest to control of nearly the entire country, were both reopening their embassies in Damascus.

Since Mr. al-Assad fled the advancing rebels on Dec. 8, the rest of the world has had to reckon with a sudden new reality in Syria: A country where nearly 14 years of civil war had left Mr. al-Assad in seemingly firm control was now in the hands of a conservative Islamist group, Hayat Tahrir al-Sham, that the United Nations, the United States, Turkey and many other countries had long designated as a terrorist organization for its early ties to Al Qaeda and the Islamic State.

Arab countries had for years been moving toward normalizing relations with Mr. al-Assad, despite his brutal treatment of his people, and Western countries, while hitting him with heavy sanctions, had grudgingly come to accept that he was there to stay. His overthrow scrambled that calculus, forcing foreign powers to decide how to deal with a largely unknown quantity that many of them had shunned as extremists for years.

Many of those powers, including the United States, European countries and Turkey, say they want to see a stable, unified Syria with an inclusive government that respects the rights of Syria’s minorities, including Shiite Muslims, Druse, Christians of various sects and Alawites, the Shiite offshoot sect that the Assad family and many of its strongest supporters belongs to.

Foreign countries have the leverage to push Syria’s new leadership toward that vision. To unlock greater flows of humanitarian aid, get suffocating economic sanctions lifted and earn international legitimacy — all required for a crippled, impoverished Syria to stabilize and rebuild — Hayat Tahrir al-Sham will need other countries to remove its designation as a terrorist group.

Ms. Kallas has said that the European Union will not lift sanctions on Syria until its new leadership shows it will protect minorities and women’s rights and disavow extremism. On Monday, she told reporters that European foreign ministers meeting in Brussels would discuss “how we engage with the new leadership of Syria and on what level we engage the leadership and, of course, what more steps are we willing to take if we see that Syria goes to the right direction.”

Individual European countries were also gradually reaching out to Damascus.

Italy, which has maintained a diplomatic presence in the Syrian capital since 2018, was the first to engage on the ground. Its ambassador was the only European representative in a meeting the Syrian transitional administration held last week with several Arab ambassadors, according to Italy’s foreign ministry.

Jean-Noël Barrot, France’s foreign minister, told France Inter radio on Sunday that a team of four French diplomats would head to Syria on Tuesday for the first time since 2012, when France and many other countries broke with Mr. al-Assad over the bloody crackdown on peaceful antigovernment protesters that instigated the civil war.

Mr. Barrot said the main goals were to establish first contact with the Syrian authorities there and to evaluate the needs of the Syrian population.

“But also to verify whether or not the initial statements made by this new authority — which were rather encouraging, which called for calm, which apparently did not commit any abuses — are actually being followed up on the ground,” Mr. Barrot added.

The quickening diplomatic engagement reflected the winners and losers in the new Syria.

Russia, a key ally of Mr. al-Assad, said over the weekend that it had evacuated some staff members from its embassy in Damascus, though the embassy confirmed that its ambassador was staying.

But Turkey, which has long had tacit links to Hayat Tahrir al-Sham and has emerged as an important go-between for the group and other foreign governments, raised its flag over its embassy in Damascus on Saturday for the first time in 12 years. And Qatar, which like Turkey has maintained a relationship with the group and supports Islamist groups around the Middle East, likewise sent a diplomatic delegation to Syria to reopen its embassy there, its foreign ministry said in a post on X on Sunday evening.

Mr. al-Shara, who has long craved international legitimacy for Hayat Tahrir al-Sham, appears attuned to concerns about whether his group is ready to lead. According to the Telegram post announcing his meeting with Mr. Pedersen, he said it would be important to secure economic and political support for creating a safe environment for the millions of Syrian refugees in the Middle East, Europe and elsewhere to return.

“Leader al-Shara pointed out the need to implement these steps with great care and high precision without haste and under the supervision of specialized teams, so that they are achieved in the best possible way,” it said.

Aurelien Breeden contributed reporting from Paris; Emma Bubola from Rome; Jacob Roubai from Beirut, Lebanon; and Natalia Vasilyeva from Istanbul.

The “successive strikes” along the Syrian coast — home to Russian naval bases — amounted to “the most violent strikes in the area” since 2012, according to the Observatory. It said there were 18 airstrikes, which were particularly powerful because they were consecutive and detonated missiles in warehouses, leading to secondary explosions.

The Israeli military declined to comment on the strikes. Israeli officials have previously said that the campaign in Syria is an effort to keep military equipment out of the hands of “extremists,” after an alliance of rebel groups ousted the Assad regime earlier this month. There were no immediate reports of casualties from the latest strikes, the Observatory said.

Israel has struck Syria more than 450 times since the collapse of the Assad government, according to the Observatory, destroying Syria’s navy and dozens of air bases, ammunition depots and other military equipment.

Israel’s military has also seized and occupied an expanse of territory in Syria over the de facto border between the two countries, including on the Syrian side of the strategic Mt. Hermon. Israel has given no timeline for its departure, apart from saying that it would stay until its security demands were met.

On Sunday, the Israeli government unanimously approved plans by Prime Minister Benjamin Netanyahu to expand settlements in the Israeli-occupied Golan Heights, part of an $11 million scheme to double the population in the area. The move was necessary, the prime minister’s office said, because a “new front” had opened up on Israel’s border with Syria after the fall of the Assad government.

Israel seized the Golan Heights during the Arab-Israeli War of 1967 and it is considered illegally occupied under international law.

The head of the group leading the rebel coalition that now governs Syria, Ahmed al-Shara, said in an interview on Saturday with Syria TV, a pro-opposition channel, that Israel was using pretexts to justify “unwarranted” territorial seizures in Syria.

Still, he said, Syria could not afford any further conflict and was instead focused on diplomatic solutions.

“Syria’s war-weary condition, after years of conflict and war, does not allow for new confrontations,” Mr. al-Shara said. “The priority at this stage is reconstruction and stability, not being drawn into disputes that could lead to further destruction.”

Gabby Sobelman and Vivian Yee contributed reporting.

For more than a decade, the Kurdish-led soldiers have been America’s most reliable partner in Syria, liberating cities seized by the extremist group and detaining around 9,000 of its fighters.

But Turkey, which shares a border with Syria, has long considered the Kurdish group to be its enemy. The Turkish government believes the Kurdish fighters in Syria are allied with the separatist Kurdistan Workers’ Party, or P.K.K., which has fought the Turkish state for decades.

President Recep Tayyip Erdogan of Turkey, who backs the rebel groups that toppled the Assad regime, appears eager to seize the opportunity created by the momentous political shift in Syria to pursue his own agenda against the Kurdish fighters.

Turkey’s new dominance leaves the Kurds exposed

The shape of the new Syrian government, led by the Islamist group Hayat Tahrir al-Sham, is still being determined. But American officials and Middle East analysts agree: Turkey will have an outsized influence.

That means Kurdish groups’ foothold in the northeast looks increasingly “tenuous,” said Wa’el Alzayat, a Syria expert and former American diplomat. Turkey “will have the biggest leverage in what’s happening, and will happen, in Syria for the foreseeable future,” he said.

As Hayat Tahrir al-Sham and its allies seized control from President Bashar al-Assad, “they brought with them a tide of Turkish power and influence over the future of Syria,” said Nicholas Heras, a senior analyst at the New Lines Institute.

The high stakes for the Kurds, and for Western forces determined to prevent a renewed ISIS threat, were illuminated earlier this past week. Even as Hayat Tahrir al-Sham and its allies took over, Turkish-backed rebels attacked the Syrian Democratic Forces, supported by Turkish airstrikes and artillery fire.

The commander of the Syrian Democratic Forces, Gen. Mazloum Abdi, told The New York Times he had to divert fighters who were defending the prisons that house accused ISIS members to fight off the Turkish-backed militants.

Now, Mr. Heras predicted, Arabs who had joined the Syrian Democratic Forces to fight the Islamic State could disband or defect to other rebel groups, under pressure from Turkey and Hayat Tahrir al-Sham. That would further weaken the Kurdish forces.

A best-case scenario for the Kurds, officials and experts said, might see them receive enough support from the United States to secure the territory they hold in northeast Syria. That could give them leverage with the new government in Damascus to pursue a fully autonomous state, something minority Kurds in Syria have long sought.

At worst, the Kurds could face an inflamed conflict with Turkish-backed fighters, be forced to cede control of at least some of their oil-rich territory and, if President-elect Donald J. Trump decides to withdraw U.S. troops, lose vital help on the ground.

America’s role will be pivotal

“There really needs to be some kind of cease-fire/peace agreement between the Turks and the Kurds that both sides can agree with,” said Natasha Hall, a Syria expert at the Center for Strategic and International Studies in Washington.

The Biden administration is racing to negotiate just that before it leaves office next month.

Following meetings in Turkey last week, Secretary of State Antony J. Blinken said on Saturday that “making sure that ISIS was in a box” remained an urgent priority in Syria. He said the Kurdish fighters were “playing a critical role in pursuing that mission.”

But the diplomatic balancing act he faced was clear: His meetings in Turkey included talks with the foreign minister, Hakan Fidan, who earlier last week said that “any P.K.K. extension in Syria cannot be considered a legitimate partner.”

And on Friday, Mr. Fidan pointedly cited the P.K.K. as he described efforts to keep terrorist organizations from exploiting the political chaos in Syria.

Yet there are signs that American diplomacy is having an impact. Last week, an American commander, Gen. Michael E. Kurilla, visited northeast Syria, where 900 American troops are stationed. Hours later, a cease-fire between the Kurdish forces and a Turkish-backed rebel group known as the Syrian National Army was announced in the northern city of Manbij, where the two sides have frequently clashed.

General Abdi, the Kurdish commander, said on X that the cease-fire was brokered with American help. Under the agreement, he said, Kurdish forces would withdraw from Manbij, a majority Arab city which they seized from the Islamic State in 2016 but that has since become a flashpoint among battling factions for control. But he and other Syrian ethnic Kurds are increasingly worried that their retreat from Manbij is just the beginning.

The city of Kobani could be the next flashpoint

Last Tuesday, a senior Hayat Tahrir al-Sham officer said that local tribes allied with his group had wrested control of the eastern city of Deir al-Zour from Kurdish fighters who had taken over as Mr. al-Assad’s forces collapsed just days earlier.

And in the days since, the Turkish-backed rebels have repeatedly battled with Kurdish forces in the region around the Euphrates River.

Mr. Heras, the New Lines analyst, said he thought those skirmishes could be military preparations for an invasion of Kobani, a majority Kurdish city.

The city, just south of the Turkish border, holds deep emotional significance for the Kurdish forces, who fought with American troops to reclaim it after a four-month Islamic State siege that began in late 2015.

General Abdi now appears to be bracing for a possible invasion by Turkey’s allied fighters. Mr. Heras said residents were fleeing Kobani by the thousands despite a shaky truce agreement this past week that aimed to buy time for negotiations.

“Turkey is taking advantage of the crisis in Syria to destabilize the region and seize our land, while claiming they are fighting terrorists,” said Sinam Sherkany Mohamad, the head of the Kurdish fighters’ political wing in Washington, in a statement. “But we are not terrorists, we are democratic U.S. allies.”

James F. Jeffrey, a former American ambassador to Turkey who was a chief Syria envoy during Mr. Trump’s first administration, said any invasion of Kobani would violate a 2019 agreement that the U.S. negotiated for a détente, “and whether by the Turks, or Syrian forces associated with the Turks, it makes no difference.”

In the meantime, General Abdi has sought to shore up the Kurdish fighters’ relationship with Hayat Tahrir al-Sham, saying he is seeking direct relations with the group’s leaders.

Officials and experts said Turkey may wait until its interests are locked in with the new Syrian government before deciding whether to launch a full-bore military offensive against the Kurdish forces. It may also watch to see whether Mr. Trump withdraws American troops, and how his administration deals with Mr. Erdogan, a like-minded strongman whose relationship with the United States has often been tempestuous.

Senator Lindsey Graham, Republican of South Carolina, warned in a statement on social media that he was prepared to push for economic sanctions against Turkey if it attacked the Kurdish forces, which he said would “set in motion an ISIS jailbreak.” He added: “If Turkey takes military action against Kurdish forces in Syria, it will jeopardize America’s interests dramatically.”

Safak Timur contributed reporting from Istanbul.