US citizen fighting for ISIS in Syria surrenders to US-backed forces The individual is being detained by the DoD as a known enemy combatant.

 -- A U.S. citizen fighting for ISIS surrendered to American-backed forces in Syria this week, according to the Pentagon.

The individual, who surrendered to Syrian Democratic Forces (SDF) on or around September 12th, has since been turned over to U.S. forces, said Eric Pahon, Department of Defense spokesperson.

"The U.S. citizen is being legally detained by Department of Defense personnel as a known enemy combatant," Pahon said.

A U.S. official told ABC News that the individual is still overseas, as the Department of Justice and the Trump administration determine how to move forward.

The Department of Defense has referred all questions about the incident to the State Department and Department of Justice. But during a State Department briefing on Thursday, spokesperson Heather Nauert said she had no further information on the individual's circumstances, saying anyone who would fight for ISIS is "dumb."

“It serves as a good reminder that in a nation of 330-some million people, some people will be dumb enough to go to Iraq and Syria to try to fight for ISIS," Nauert said, adding, "You can’t be very bright if you’re going over there and doing that.”

"In general, we closely monitor all reports of U.S. citizens who may have joined groups to fight with or fight against ISIS," a State Department official told ABC News. "To be clear, the U.S. government does not support U.S. citizens traveling to Iraq or Syria to join the ongoing conflict. Any private U.S. citizens/civilians who may have traveled to Iraq or Syria to take part in these activities are neither in support of nor part of U.S. efforts in the region."

According to a 2015 report by the House Homeland Security Committee, intelligence officials estimate that more than 250 Americans have either traveled to Syria or Iraq, or attempted to travel to Syria or Iraq, to join ISIS.