Syria's Assad is now in Moscow, Russian state media says

Geir Otto Pedersen, the United Nations' special envoy for Syria, said in a statement Sunday that the fall of President Bashar al-Assad's government "marks a watershed moment" in the country's history after "nearly 14 years of relentless suffering and unspeakable loss."

"This dark chapter has left deep scars, but today we look forward with cautious hope to the opening of a new one -- one of peace, reconciliation, dignity and inclusion for all Syrians," Pedersen said in a statement.

"The challenges ahead remain immense and we hear those who are anxious and apprehensive," Pedersen added. "Yet this is a moment to embrace the possibility of renewal. The resilience of the Syrian people offers a path toward a united and peaceful Syria."

Pedersen called for "stable and inclusive transitional arrangements" to smooth the transfer of power on the path to "a unified Syria, with its sovereignty, independence and territorial integrity, in a way that can receive the support and engagement of the entire international community."

Syrian refugee communities abroad are celebrating the fall of President Bashar al-Assad's government after a week-long lightning rebel offensive.

The civil war that erupted in 2011 has forced around 12 million Syrians -- more than half of the country's 2011 population of around 22 million -- from their homes. Around 5.4 million of these were still living abroad as of late 2022, per United Nations data.

Crowds gathered in cities in Turkey and Lebanon on Saturday night into Sunday morning to celebrate the news of Damascus falling to rebel forces advancing from the north and south.

Photos showed large crowds gathered in central Istanbul waving Syrian opposition and Turkish flags to celebrate the rebel victory.

Turkey and Lebanon host the largest number of Syrian refugees. Germany, Jordan, Iraq, Egypt and Sweden are also major centers for displaced Syrians.

President-elect Donald Trump said in a post to Truth Social early Sunday that Russia "lost all interest in Syria because of Ukraine, where close to 600,000 Russian soldiers lay wounded or dead, in a war that should never have started, and could go on forever."

Syrian President Bashar al-Assad "is gone," Trump said, adding: "He has fled his country."

Russian President Vladimir Putin "was not interested in protecting him any longer," Trump wrote. "There was no reason for Russia to be there in the first place."

Rebel forces claimed on Sunday morning that Assad had left Damascus. His current whereabouts are unknown.

"Russia and Iran are in a weakened state right now," Trump added, citing Moscow's struggles in Ukraine and recent Israeli military successes against Iran and its regional proxy forces across the Middle East.

White House National Security Adviser Jake Sullivan on Saturday acknowledged "concern" over the jihadist origins of the Syrian rebel group leading the campaign that appears to have unseated President Bashar al-Assad.

Speaking at the Reagan National Defense Forum event, Sullivan said that the history of the Islamist Hayat Tahrir al-Sham group -- which has been designated a terrorist group by the U.S. -- "is a concern," noting that elements of the group are affiliated with organizations "that have American blood on their hands."

Sullivan said the U.S. is focused on making sure the fighting in Syria does not lead to a resurgence of ISIS, which broke off from al-Qaida and took control of large parts of Iraq and Syria between 2013 and 2019.

Sullivan said the U.S. is also concerned with making sure friends and allies in the region -- particularly Israel, Jordan and Iraq -- remain secure and that the fighting does not lead to a humanitarian catastrophe, particularly for religious and ethnic minorities in Syria.

-ABC News' Jack Moore