Reports that an American is in the custody of the mostly Kurdish Syrian Democratic Forces were not disputed by U.S. officials, but they withheld further comment.
In a video briefing to the Pentagon from Baghdad, Army Col. Ryan Dillon, a spokesman for Combined Joint Task Force-Operation Inherent Resolve, said, "We have seen the same reports" on an American being held by the SDF.
He deferred questions to the State Department, which has yet to comment.
Dillon said he could not immediately recall any other incident involving an American citizen in Iraq or Syria "either being captured or surrendering" after joining up with the Islamic State of Iraq and Syria.
He said that five ISIS fighters, clutching leaflets urging surrender dropped by the U.S. on Raqqa in eastern Syria, had turned themselves into the SDF but that a U.S. citizen was not among them.
Another U.S. military spokesman, Army Maj. Earl Brown, told the Daily Beast, "We are aware of the report that a U.S. citizen believed to be fighting for ISIS surrendered to Syrian Democratic Forces on or about Sept. 12."
He deferred questions to the Justice Department and added, "The coalition's mission is to defeat ISIS in Syria and Iraq, and we will pursue ISIS fighters regardless of nationality."