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

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

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

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

The U.S.-backed Syrian Democratic Forces, which control swathes of northeast Syria, referred questions to the U.S.-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 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.

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

U.S. forces first deployed into Syria during the Obama administration's campaign against Islamic State, partnering with a Kurdish-led group called the Syrian Democratic Forces.

While Islamic State is now a shadow of the group that ruled over a third of Syria and Iraq in a caliphate declared in 2014, hundreds of fighters are still camped in desolate areas where neither the U.S.-led coalition nor the Syrian army, with support from Russia and Iranian-backed militias, exert full control.

Thousands of other Islamic State fighters are in detention facilities guarded by Kurdish-led Syrian Democratic Forces, America's key ally in the country.

U.S. officials say that Islamic State could still regenerate into a major threat.

The threats from Iran-backed militia to U.S. forces are a reminder of the complex geopolitics of Syria, where Syrian President Bashar al-Assad counts on support from Iran and Russia and sees American troops as occupiers.