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

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

The Israel Defense Forces said Sunday it deployed troops into a key buffer zone along the Syrian border following the fall of Damascus to rebel forces and the collapse of President Bashar al-Assad's government.

"In light of the events in Syria and in accordance with the assessment of the situation and the possibility of armed men entering the buffer zone, the IDF deployed forces in the buffer zone and at several points necessary for defense," the IDF wrote on X.

The buffer zone is known as the Area of Separation and was established as part of Israel's ceasefire agreement with Syria in 1974. In November, the United Nations accused Israel of "severe violations" of the agreement by building new trenches and earth berms in the buffer zone.

Israeli forces deployed into the area is "in order to ensure the security of the Golan Heights settlements and the citizens of Israel," the IDF added, referring to the border plateau area occupied by Israel since 1967 but still claimed by Syria. Israel unilaterally annexed the area in 1981, a move recognized by the U.S. in 2019.

"We will emphasize that the IDF does not intervene in the events taking place in Syria," the IDF added.

"The IDF will continue to operate as long as necessary in order to preserve the buffer zone and protect the state of Israel and its citizens," it said.

-ABC News' Jordana Miller

In a stunning turn, over the last few hours, it appears that the Assad regime has fallen after a rebel advance caught Syrian forces by surprise when its backers in Iran, Russia, and Hezbollah were weak and distracted.

The Syrian Prime Minister Mohammed Ghazi 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.

Rebel forces claim that Assad has left the capital and, while that is not confirmed, they have clearly taken control of many parts of the city.

-ABC News' Kirit Radia

President Joe Biden is "closely monitoring the extraordinary events in Syria," White House National Security Council spokesperson Sean Savett said in a post on X.