Rebel forces in Syria said they were advancing into the capital Damascus after capturing four other cities in the past 24 hours as a lightning advance by insurgents continued, threatening President Bashar al-Assad’s grip on power.

In the early hours of Sunday morning local time, the Islamist Hayat Tahrir al-Sham group, or HTS, said on the Telegram messaging app, that rebel forces had entered Damascus and reached Sednaya prison, a government facility dubbed the “human slaughterhouse” by the human rights group Amnesty International.

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

Earlier, the group claimed it had taken control of the city of Homs, in the north-central part of the country on Saturday.

HTS leader Abu Mohammed Al-Jolani released a video claiming that government forces withdrew without a fight there, and the rebel group later released a statement, “Our eyes are watching the capital, Damascus.”

Overall, the insurgent group claimed credit for taking three other Syrian cities over the past day — including Daraa, Queinetra and Sweida — in a series of rapid advances by opposition fighters that have 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 was forced to deny 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.

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.

President-elect Donald 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.

“This is a remarkable series of developments,” Javed Ali, associate professor at the University of Michigan’s Ford School of Public Policy, described the rebel advances, during an appearance on ABC News Live.

He called it a “catastrophic loss” to 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.”