"Syrian Democratic Forces turned over to US forces an American citizen who surrendered to the SDF on or around September 12," US Marine Corps Major Adrian Rankine-Galloway told CNN, referring to the US-backed Kurdish led group fighting ISIS in Syria.
"The US citizen is being legally detained by Department of Defense personnel as a known enemy combatant," he added.
Rankine-Galloway referred CNN to the Department of Justice for additional information.
The Department of Justice and the FBI declined to comment.
A US military official had previously told CNN that an American citizen had appeared to have surrendered to Kurdish elements of the Syrian Democratic Forces.
Earlier on Thursday a spokesperson for the US-led coalition fighting ISIS would not confirm that a US citizen was in the custody of the SDF, referring questions to the US State Department.
"We have seen those same reports and the SDF as our partner has taken an oath to make sure that any fighters that they capture that they maintain them and bring them to the proper authorities. In this case the proper authorities would be the equivalent of the department of state in the country," Col. Ryan Dillon told reporters at the Pentagon via a video conference from Baghdad.
"If this was a US citizen it would be the Department of State to find out the updates on that particular person," Dillon added.
A State Department official told CNN that the department was aware of the reports but said that they "have no information to share at this time."
The Daily Beast was first to report
that a US citizen had been detained by the SDF.
This isn't the first US citizen fighting for ISIS to be detained by US allies in the region. In March 2016, a US citizen was captured
in Iraq by US-backed Kurdish Peshmerga forces.
The number of US citizens traveling to Iraq and Syria to join ISIS is thought to be much smaller than other countries and regions such as Western Europe and coalition officials have noted that far fewer foreign fighters have joined ISIS in recent months as the terror group suffers setbacks on the battle field.
ISIS' recent defeats have also caused an increase in the number of fighters surrendering to the SDF with Dillon telling reporters that five ISIS fighters, including a local commander, had surrendered this week alone.
This story has been updated.