Syrian Democratic Forces fighters in Deir al-Zour, Syria, on Tuesday. (Rodi Said/Reuters)

An American citizen fighting with the Islamic State in Syria has been captured by local forces and turned over to the U.S. military in that country, the Pentagon said Thursday.

"Syrian Democratic Forces turned over an American citizen who surrendered to SDF on or around Sept. 12 to U.S. forces," said a Pentagon spokesman, Marine Maj. Adrian Rankine-Galloway. "The U.S. citizen is being legally detained by Department of Defense personnel as a known enemy combatant. We refer further questions on this matter to the Department of Justice."

The detainee was not identified, and further details of his surrender and detention were not made available. The Justice Department did not respond to requests for information, and the State Department said it had none.

The Syrian Democratic Forces, composed of Kurds and Arabs, are the main body of local U.S. allies fighting against the Islamic State in Syria. U.S. forces are advising them and providing air support and other backing as they push into militant territory.

U.S. officials have estimated that 40,000 foreign fighters have flowed to Iraq and Syria to join the Islamic State and other groups since the conflict there began. The vast majority have been from the Middle East and North Africa, with substantial numbers from Europe and former Soviet republics with large Muslim populations.

Then-FBI Director James B. Comey estimated in late 2015 that more than 200 Americans had tried to join the militants in Syria or provided support to those who joined. According to the New America think tank, 44 succeeded in reaching the Islamic State or other groups in Syria.

Foreign fighters captured in Iraq have been held by the Iraqi military, which has said it would try them — this week it sentenced a Russian-born Islamic State fighter to death. European governments and others have said that any of their nationals captured should be returned for trial at home.

A Virginia man, Mohamad Khweis, was convicted in federal court in June of supporting terrorism after he surrendered in early 2016 to Kurdish forces in Iraq who turned him over to the American military there.

Khweis, due to be sentenced next month, said he had traveled undetected from Fairfax County to join the Islamic State in Syria but at the time of his capture had deserted the militants and was trying to make his way home.

The Pentagon's referral to the Justice Department indicated that the new American detainee would also be turned over to federal courts.

The prisoner whose detention was announced Thursday is the first known to have been captured under the Trump administration. His capture was first reported by the Daily Beast.

Although President Trump said during his presidential campaign that he would be "fine" with sending U.S. citizens captured on the terrorism battlefield to the U.S. detention facility at Guantanamo Bay, Cuba, to be tried by military commissions, such proceedings are, by statute, authorized only for foreigners.

Trump and members of his administration have said they hoped to send newly captured enemy combatants to the prison. In March, Attorney General Jeff Sessions said it was "just a very fine place for holding these kind of dangerous criminals."

"In general, I don't think we're better off bringing these people to federal court," where they are represented by "court-appointed lawyers" and given "discovery rights to find out our intelligence," Sessions said.

But the first foreign terrorism suspect to face trial under the administration was brought to the United States for civilian trial. Ali Charaf Damache, an Algerian-born Irish citizen accused of conspiring to support terrorists, was captured in 2015 in Spain and extradited here early this year on a 2011 indictment from the Eastern District of Pennsylvania.

Dan Lamothe contributed to this report.