A man believed to be an American citizen who was fighting with Islamic State militants has surrendered in Syria and is being held in American custody.

Pentagon spokesman Major Adrian Rankine-Galloway said the man, who surrendered to the Syrian Democratic Forces and was turned over to the US military force in Syria, is being legally detained as a known enemy combatant.

The decision to legally detain the man as a “known enemy combatant” comes as the Trump administration has been working to craft a detention policy which could determine whether the US will resume sending detainees to the military prison at Guantanamo Bay, Cuba.

US flag
(PA)

Former president Barack Obama did not send any new detainees to Guantanamo and currently there are only about 40 people held there.

Donald Trump has expressed a willingness to send new detainees to the site, but so far has not.

It is not clear if the detainee, who has not been identified, and the American forces holding him are still in Syria, but the plan is to take the man into Iraq, where he would then be turned over to the US State Department or Justice Department.

Washington said it is working out what it will do with the man if he is confirmed as an American.

If his citizenship is confirmed, he would be the second known American taken into custody for fighting with IS insurgents.

Trump administration officials have said there are three viable options today for taking enemy combatants off the battlefield: they can be killed, they can be apprehended and released after a few weeks, which could involve interrogation, or they can be captured and handed over to a third party.

The third approach causes concern about the possibility that detainees outsourced to a third party could be treated inhumanely.

Suspects accused of terror-related offences, including Americans, can also be dealt with in US courts.