In a stunning turn, the regime of Syrian President Bashar al-Assad appears to have fallen after rebels advanced into the capital Damascus, catching government forces by surprise 10 days after a lightning advance by insurgents first began.

Early Sunday morning local time, the rebel military operations command for the Islamist Hayat Tahrir al-Sham group, or HTS, claimed the president was no longer in the capital, writing: “We declare the city of Damascus free of the tyrant Bashar al-Assad.”

The Russian Foreign Ministry said in a Sunday morning statement that Syrian President Bashar al-Assad “decided to leave the presidential post and left the country, giving instructions to transfer power peacefully.” Russia and Iran were the two most important foreign backers of Assad’s government. Assad’s whereabouts remain unknown.

Videos on social media showed Syrian forces abandoning their posts and melting away. There were also reports of Syrian military troops surrendering in other cities.

A few hours earlier, the rebel group first announced it had advanced directly into the capital city, reaching Sednaya prison, a government facility dubbed the “human slaughterhouse” by the human rights group Amnesty International.

“Our forces have begun entering the capital Damascus,” HTS said in one message. In a follow-up statement, the group said it was “the end of the era of injustice of Sednaya prison.”

The Syrian prime minister, Ghazi al-Jalali, released a video saying the government was ready to “extend a hand” to the opposition and hand over its functions. Rebel leaders have instructed their fighters not to approach Syrian government sites, an apparent attempt to quell any fears in the city and await a formal handover in the morning.

President Joe Biden was “closely monitoring the extraordinary events in Syria,” White House National Security Council spokesperson Sean Savett said in a statement.

Speaking at a defense conference earlier in the day, before rebels advanced into the capital, U.S. National Security Adviser Jake Sullivan said the speed and scale of their rapid advance came, in part, because Assad’s chief backers — Iran, Russia and Hezbollah — had all been “weakened and distracted,” in recent months.

That has left Assad “basically naked,” Sullivan said at the Reagan National Defense Forum in California. “His forces are hollowed out.”

President-elect Donald Trump, meanwhile, said in a post to Truth Social early Sunday that Assad “is gone” and had “fled his country.” Russia, Trump added, “was not interested in protecting him any longer.”

The incoming president said Moscow and its partners in Iran had been weakened by events in Ukraine and Israel’s recent military successes across the Middle East.

Lighting advance by rebel groups

Overall, the insurgent group claimed credit for taking over four Syrian cities in 24 hours — Homs, Daraa, Queinetra and Sweida — in a series of rapid advances by opposition fighters that was largely been met with little resistance from government forces.

Throughout the day Saturday, as the rebel forces were on the move, the Syrian army withdrew from much of southern Syria, leaving more areas of the country, including two provincial capitals, under the control of the rebels, according to the military and an opposition war monitor.

In a statement released earlier Saturday, the Syrian government at first denied Assad had fled the country, issuing a statement calling media reports to the contrary “rumors and false news.”

U.S. intelligence had been preparing for Assad’s front line to collapse under the press of rebel forces, and the U.S. had intelligence that Assad’s family had left the country for Moscow.

Earlier this week, government forces withdrew from Hama, Syria’s fourth largest city, which sits between the capital Damascus to the south and Aleppo — Syria’s second city — to the north.

Aleppo fell to the lightning rebel offensive on Nov. 29. Hama was one of the few major cities that did not fall to anti-government forces following the unsuccessful 2011 revolution against Assad’s rule.

Who are the Syrian rebels?

The rebel offensive is being waged by HTS and a collection of Turkish-backed Syrian militias known as the Syrian National Army.

HTS, which has its roots in al-Qaida, is considered a terrorist organization by the U.S.

HTS is led by Abu Mohammad al-Jolani, who in recent days has been trying to distance himself from his jihadist past. Jolani was born in Saudi Arabia to Syrian parents from the Israeli-occupied Golan Heights and and raised in Damascus.

Jolani said in an interview with PBS in 2021 that he fought for al-Qaida in Iraq during the American occupation. Jolani said he was arrested by American forces and held for more than five years in several detention facilities, including the infamous Abu Ghraib and Camp Bucca prisons.

Jolani said his ideology has since evolved. HTS said it has severed ties with al-Qaida in recent years and sought to remake itself by focusing on promoting civilian government and military action, according to The Associated Press.

If Assad flees the country and HTS gains control of Syrian government institutions, it’s unknown how they will seek to govern.

“Will they revert back more to that when it was affiliated with al-Qaida?” said Javed Ali, associate professor at the University of Michigan’s Ford School of Public Policy, during an appearance on ABC News Live.

“Or will it look more like the Taliban in Afghanistan — which is Islamist, conservative, but not, for the most part, intent on threatening its neighbors or having its country be used as a launchpad for attacks against the West.”

What officials are saying about the situation in Syria

Ali called the developments a “catastrophic loss” for Iran, which has long supported the Assad regime and a “black mark” for the Russian government, which has also propped up Assad.

Citing media reports, both countries appear to have pulled advisers and equipment out of the country, Ali said.

“If these withdrawals continue, both on the Iranian and the Russian side, it’s only basically the Syrian military and security forces left,” he said. “And much like we saw in Afghanistan in 2021, they are probably not going to be able to hold back against HTS and all the other rebel groups that are on the outskirts of Damascus right now.”

Trump weighed in on the developments in the Syrian civil war on his social media platform Truth Social on Saturday and said the U.S. should stay out of the matter entirely.

“Syria is a mess, but is not our friend, & THE UNITED STATES SHOULD HAVE NOTHING TO DO WITH IT. THIS IS NOT OUR FIGHT. LET IT PLAY OUT. DO NOT GET INVOLVED!” Trump posted.

In the post, Trump noted that Russia, which has long supported Assad’s regime, is “tied up in Ukraine” and apparently unable to intervene in Syria. Trump said Assad being forced out “may actually be the best thing that can happen” to the Russian government.

Meanwhile earlier Saturday, Russian Foreign Minister Sergey Lavrov commented on the rebel offensive in Syria, saying Russia would oppose it “in every way possible” but Russia “will actively promote the need to resume dialogue with the opposition,” meaning between the government and the rebels.