Outgoing US President Joe Biden said the United States will work with "all Syrian groups" within the framework of the United Nations to ensure a political transition after the fall of Bashar al-Assad's regime.

"We will work with all Syrian groups, and assess their words and actions, including the UN-led process, with the goal of establishing a transition away from the Assad regime and toward an independent, sovereign Syria with a new constitution," Biden said in a speech from the White House.

He added that the fall of the Assad regime represents a historic moment and an opportunity for the Syrian people to build peace in their country and for the Middle East region, stressing that his administration will support Syria's neighbors, including Jordan, Iraq, Israel and Lebanon.

Biden pointed out that the Assad regime had brutally killed thousands of innocent civilians and should be held accountable, stressing that it had refrained from engaging in a serious political process and continued to commit crimes against its people.

Biden said that his country "will not give ISIS a chance to build its capabilities, and will not allow Syria to become a safe haven for it. We will work to protect our forces in Syria and continue our mission against ISIS."

The Syrian armed opposition announced the fall of the Assad regime and the entry of its forces into the capital Damascus - at dawn on Sunday - as the culmination of a series of swift victories it achieved in the past few days in Aleppo, Hama, and then Homs.