President Joe Biden cheered the toppling of Syria's dictatorship in remarks on Sunday, but he warned about the uncertainty during the country's political transition.

"It's a moment of historic opportunity for the long-suffering people of Syria to build a better future for their proud country," he said from the White House's Roosevelt Room. "It's also a moment of risk and uncertainty. As we turn to the question of what comes next, the United States will work with our partners and the stakeholders in Syria to help them seize an opportunity to manage the risk."

The U.S. will support Syria's neighbors, including Jordan, Iraq, Lebanon and Israel, during the transition period and send senior officials to the region, Biden said. He also pledged to continue the mission against ISIS and to ensure the security of detention facilities in Syria that hold ISIS fighters.

"We're clear-eyed about the fact that ISIS will try to take advantage of the new vacuum to reestablish its capability and create a safe haven. We will not let that happen," Biden said.

He added that U.S. forces conducted a dozen precision air strikes in Syria on Sunday, targeting ISIS camps and operatives.

But he noted that it isn't just ISIS that poses a threat. Biden said the U.S. will stay vigilant, adding that some of the rebel groups that took down Assad's regime have their own records of terrorism and human rights abuses.

"They're saying the right things now, but as they take on greater responsibility, we will assess not just their words but their actions," he said.

The U.S. will also help "all Syrian groups" establish an independent, sovereign government. That pledge will include humanitarian aid to help Syria after 13 years of civil war.

Biden also said that his administration is committed to returning Austin Tice to his family. Tice, an American journalist, was abducted in Damascus in 2012 and detained by the Syrian government. Biden said the U.S. thinks Tice is alive.

Syrian rebel forces seized the capital of Damascus on Sunday. Russia's Foreign Ministry said President Bashar al-Assad has resigned and left the country, ending his family's decades-long dictatorship in Syria.

"We're not sure where he is, but there's word that he's in Moscow," Biden said, adding later that Assad should be held accountable.

Assad assumed leadership in the country in 2000, following his father's death. Assad's allies included Russia, Iran and the Lebanese militant group Hezbollah. Biden said all three allies are "far weaker" now than they were when he took office.