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The United States will work with partners and stakeholders in Syria to help seize the opportunity that has presented itself after rebel fighters' stunning overthrow of Syrian President Bashar al-Assad, President Joe Biden said on Sunday.

"At long last, the Assad regime has fallen," said Biden, commenting on a fast-moving string of events in Syria that caught some in the White House off guard.

Biden and his top national security aides took stock of a new-look Middle East that may be emerging after rebel forces ousted Assad in little more than a week.

Biden said Syria is in a period of risk and uncertainty and that it is the first time in years that neither Russia nor Iran nor the Hezbollah organization held an influential role in Syria.

"For years, the main backers of Assad have been Iran, Hezbollah and Russia. But over the last week, their support collapsed - all three of them - because all three of them are far weaker today than they were when I took office," said Biden, who became president in 2021.

He said Syria faced a period of "risk and uncertainty" and that the United States would help where it can.

"As we all 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," Biden added.

But a senior US official cautioned that the United States did not plan to offer a blueprint for Syria going forward.

"The future will be written by Syrians," the official told reporters.

The United States was watching closely statements from the HTS rebel organization that tossed out Assad. The group is "saying the right things" at this stage but that it was too early to say what was going to happen in Syria, the official added.

The United States will work to ensure the safety of chemical weapons stockpiles in Syria, the senior US official said, without elaborating except to say there would not be a role for US troops on the ground in this effort.

A second senior US official told Reuters that the United States would likely keep its estimated 900 troops in eastern Syria as a hedge against ISIS operatives.

The future of Russia's two main bases and some smaller ones in Syria was unclear. The bases have been used to back Assad and give Russia a footprint in the region.

Assad is in Moscow with his family after Russia granted them asylum on humanitarian grounds, a Kremlin source told Russian news agencies on Sunday, and a deal has been done to ensure the safety of Russian military bases.

Biden said US forces on Sunday conducted dozens of precision strikes within Syria targeting camps and operatives of Islamic State to keep the militant group from reasserting itself.

"It's a moment of historic opportunity for the long suffering people of Syria to build a better future for their proud country. It's also a moment of risk and uncertainty," Biden said.