Turkish police say they arrested a US citizen at Istanbul airport last month where they suspect he handed over his own passport to a Syrian national.
Officers say the Syrian man, identified as RS, was trying to fly to Germany using the document.
Police initially said the American man worked at the US consulate in Beirut and had $10,000 (£7,400) in cash when he was arrested.
But, on Wednesday, the US denied that the man was a diplomat.
“We are aware of the detention of a US citizen in Turkey,” the State Department said in a statement. “The individual is not a US diplomat. We are providing appropriate consular services.”
CCTV footage from the incident on 11 November shows the men swapped clothes at the airport, Turkish police said.
The Syrian is facing charges of forging an official document and has been released on bail.
The American, identified only with the initials DJK, remains in detention, according to Turkish police.
Diplomatic relations between the US and Turkey have been strained in recent years, including a recent row over comments Western powers made about Osman Kavala – a philanthropist jailed over protests and a military coup attempt in 2016.