Washington DC - US President Joe Biden on Sunday said deposed Syrian leader Bashar al-Assad should be "held accountable" but called the nation's political upheaval a "historic opportunity" for Syrians to rebuild their country.

In the first full US reaction to Assad's overthrow by an Islamist-led coalition of rebel factions, Biden expressed measured optimism.

However, he also warned that Washington would "remain vigilant" against the emergence of terrorist groups.

"The fall of the regime is a fundamental act of justice," Biden said, speaking from the White House.

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"It's a moment of historic opportunity for the long-suffering people of Syria."

Asked by reporters what should happen to the deposed president, who reportedly has fled to Moscow, Biden said that "Assad should be held accountable."

Biden, who is set to step down in January and make way for Republican Donald Trump's return to power, said Washington will assist Syrians in rebuilding.

"We will engage with all Syrian groups, including within the process led by the United Nations, to establish a transition away from the Assad regime toward independent, sovereign" Syria "with a new constitution," he said.

Biden cautioned that hardline Islamist groups within the victorious rebel alliance will be under scrutiny, however.

"Some of the rebel groups that took down Assad have their own grim record of terrorism and human right abuses," Biden said.

The US had "taken note" of recent statements by rebels suggesting they had since moderated, he said, but cautioned, "We will assess not just their words, but their actions."

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Biden said Washington is "clear-eyed" that the Islamic State extremist group, often known as ISIS, "will try to take advantage of any vacuum to reestablish" itself in Syria.

"We will not let that happen," he said, adding that on Sunday alone, US forces had conducted "a dozen" strikes against ISIS inside Syria.

Earlier, Biden met with his national security team at the White House to discuss the situation.

Assad's reported departure comes less than two weeks after the Islamist Hayat Tahrir al-Sham (HTS) group challenged more than five decades of Assad family rule with a lightning rebel offensive that broke long-frozen frontlines in the country's civil war.

They announced Sunday they had taken Damascus and that Assad had fled, prompting celebrations around the country and a ransacking of Assad's luxurious home.

A Kremlin source told Russian news agencies that the deposed leader was now in Moscow along with his family.

The US military has around 900 troops in Syria and 2,500 in Iraq as part of the international coalition that was established in 2014 to help combat the Islamic State jihadist group.

It has regularly struck targets in the country including those linked to Iranian-backed militias. Tehran was a major backer of Assad's government.

Biden also confirmed that US authorities believe the American journalist Austin Tice, who was abducted in Syria in 2012, still lives.

"We believe he's alive," Biden said, but the US has yet "to identify where he is."