Speaking at the White House, Biden said the US believes Austin Tice is alive, but they must pinpoint his location in the war-torn country.

Mr Tice, a freelance journalist, is thought to have been taken captive close to Damascus on 14 August 2012 while he was covering the country’s civil war.

On Sunday, rebel fighters seized the Syrian capital in the culmination of a lightning offensive launched two weeks ago. Syrian President Bashar al-Assad has fled the country.

Biden said Assad’s exit was a “fundamental act of justice” after decades of repression, but also “a moment of risk and uncertainty” for the Middle East.

“We are mindful that there are Americans in Syria,” Biden said on Sunday, “including those who reside there, as well as Austin Tice, who was taken captive more than 12 years ago.

“We remain committed to returning him to his family.”On his way out of the room, Biden turned to answer a question from the media about Tice.

“We believe he’s alive,” said the president. “We think we can get him back, but we have no direct evidence of that yet.”

The president added: “We have to identify where he is.”Mr Tice, 43, was last seen in a video, blindfolded and in apparent distress, posted online weeks after his capture.

While no government or group claimed responsibility for his disappearance, US officials soon said they believed that the former US Marine was being held by the Syrian government.

Mr Tice’s sister, Abigail Edaburn said on Friday that they believe he is still in Syria.”We don’t know the exact circumstances of the place that he’s being held, but we do know it is in Syria and that he is healthy and well,” she said.

“I don’t know how much I can say, but there have been independent, trusted sources that have been able to verify this information,” she added.

The FBI said in a statement on Sunday that a $1m reward was still on offer for information that leads to Mr Tice’s “safe location, recovery and return”.

The US has about 900 troops in Syria, and Biden said on Sunday he planned for those forces to remain.

The president also said US forces had conducted “dozens” of what he called “precision air strikes” on Sunday against Islamic State group camps and operations in eastern Syria.

President-elect Donald Trump said on Saturday that the US should not intervene militarily in Syria. “THIS IS NOT OUR FIGHT,” he wrote on social media.

The Syrian opposition that brought down Assad is led by Hayat Tahrir al-Sham (HTS), which has been designated a terrorist group by the Biden administration.

The US, UK, UN and others consider HTS to be an al-Qaeda affiliate, though HTS says it broke off ties with the Sunni Islamist organisation years ago.