Biden calls for Assad to be 'held accountable'

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 also warned that Washington will "remain vigilant" against the emergence of terrorist groups, announcing that US forces had just conducted fresh strikes against militants from the Islamic State organization.

"The fall of the regime is a fundamental act of justice," Biden said from the White House. "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 "Assad should be held accountable."

Biden -- 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 government toward independent, sovereign" Syria "with a new constitution," he said.

However, Biden cautioned that hardline Islamist groups within the victorious rebel alliance will face scrutiny.

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

The United States 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."

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 US forces on Sunday conducted strikes against ISIS inside Syria.

The US military said warplanes hit Islamic State operatives and camps.

Strikes were carried out against "over 75 targets using multiple US Air Force assets, including B-52s, F-15s, and A-10s," the US Central Command said.

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 Syria's civil war.

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

A Kremlin source told Russian news agencies the deposed leader and family were now in Moscow.

Secretary of State Antony Blinken echoed Biden's optimism, saying that "after 14 years of conflict, the Syrian people finally have reason for hope."

He also reiterated the need for justice and vigilance.

"We will support international efforts to hold the Assad regime and its backers accountable for atrocities and abuses perpetrated against the Syrian people, including the use of chemical weapons."

One outstanding concern is the fate of the Syrian government's apparent stockpile of such weapons, and whether they will fall into rebel hands.

"We are taking very prudent measures in that regard," a senior US administration official told reporters Sunday.

The US military has "good fidelity" on the location of chemical weapons, the official said, stressing that "we're doing everything we can to prudently ensure that those materials... obviously are either not available to anyone or are cared for."

Damascus has been accused on numerous occasions of using chemical weapons, charges it denies.

The US military has around 900 troops in Syria and 2,500 in Iraq as part of the international coalition 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 US authorities believe the American journalist Austin Tice, who was abducted in Syria in 2012, is still alive, but they have yet "to identify where he is."