Twenty-two US service members were injured in a helicopter “mishap” in northeast Syria on Sunday, the US military said late on Monday, without disclosing the cause of the incident or detailing the severity of the injuries.

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.

Officials at US Central Command did not immediately respond to requests for further information.

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The US-backed Syrian Democratic Forces, which control swathes of northeast Syria, referred questions to the US-led coalition under which American troops are deployed in the zone.

The autonomous Kurdish-led administration which governs the area and the central Syrian government in Damascus did not immediately respond to requests for comment.

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

In March, 25 US troops were wounded in strikes and counter-strikes in Syria, which also killed one US contractor and injured another.

US forces first deployed into Syria during the Obama administration’s campaign against IS, partnering with a Kurdish-led group called the Syrian Democratic Forces.