In a Twitter post Tuesday, the US military said it was investigating the cause of the crash and there was no indication the helicopter had been hit by a hostile attack.

The US military's Central Command said 10 service members were evacuated to higher-level care facilities outside the region.

Central Command, which oversees US troops in the Middle East, said no enemy fire was reported but added that the cause of the incident was under investigation.

There are about 900 U.S. personnel deployed to Syria, most of them in the east, as part of a mission fighting the remnants of Islamic State. American troops there have come under repeated attacks in recent years by Iran-backed militia.