Twenty-two US servicemen were injured in a helicopter "mishap" in Syria on Sunday, the US military said late Monday, without disclosing the cause of the incident.

The US military's Central Command said 10 service members were evacuated to higher-level care facilities outside the region, without detailing the severity of the injuries.

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

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 US-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.

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

The threats from Iran-backed militia to US 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.

Reporting by Reuters