Syria denies kidnapping or holding US journalist Austin Tice missing for 10 years
Austin Tice, a former US Marine, was kidnapped in August 2012 aged 31 while reporting freelance in Damascus on the uprising against Syrian President Bashar al-Assad.
His family believes he is alive and still being held in Syria. The identity of Tice's captors is not known, and there has been no claim of responsibility for his abduction.
The Syrian Foreign Ministry described Tice as a US serviceman and denied the government was holding him or any other US citizen.
"These are baseless allegations," the statement said.
Issuing a statement on the 10th anniversary of Tice's captivity, Biden said last Wednesday his administration had "repeatedly asked the government of Syria to work with us so that we can bring Austin home".
Biden said Washington knew "with certainty" that Tice had at times been held by Syria's regime.
Washington suspended its diplomatic presence in Syria in 2012 at the onset the country's civil war.
US Secretary of State Antony Blinken said Biden's special envoy for hostage affairs, Roger Carstens, would "continue to engage with the Syrian government" in coordination with the White House.
The families of hostages and detainees have begun to collectively raise their voices to urge Biden to prioritise the issue and take steps such as arranging further prisoner swaps with foreign governments.