Washington, United States - 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, speaking 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 that "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 regime toward independent, sovereign Syria with a new constitution," he said.

However, Biden cautioned that hardline Islamist groups within the victorious rebel alliance will be under 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

The US military said the strikes were conducted by warplanes against 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 on social media.

Earlier, Biden met with his national security team at the White House to discuss the crisis. 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 capital Damascus and that Assad had fled prompting celebrations nationwide and a ransacking of Assad's luxurious home. A Kremlin source told Russian news agencies that deposed leader was now in Moscow along with his family.