Washington: The US military reported late on Monday that 22 US service members were hurt in a helicopter "mishap" in northeast Syria on Sunday. The cause of the incident or the seriousness of the injuries were not disclosed.

10 service members, according to the US Central Command, were evacuated to higher-level medical facilities outside the area.

No enemy fire was reported, but Central Command, which is in charge of US forces in the Middle East, added that the incident's cause was being looked into.

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Requests for more information were not immediately answered by US Central Command officials.

About 900 US personnel have been sent to Syria, the majority of them in the east, as part of a mission to combat the Islamic State's remnants. Iran-backed militias have frequently attacked American troops there in recent years.

In March, strikes and counterattacks in Syria resulted in the wounding of 25 US soldiers as well as the death and injury of one US contractor.

During the Obama administration's fight against Islamic State, US forces entered Syria for the first time in cooperation with the Syrian Democratic Forces, a Kurdish-led organisation.

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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, can fully exert control, despite the fact that the Islamic State is a shadow of the organisation that once ruled over a third of Syria and Iraq in a caliphate declared in 2014.

The Syrian Democratic Forces, which are led by Kurds and are America's main ally in the region, are guarding detention centres where thousands of additional Islamic State fighters are being held. According to US officials, Islamic State could still resurge as a serious threat.

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Threats from militias supported by Iran to US forces serve as a reminder of Syria's complicated geopolitical situation, where Syrian President Bashar Assad relies on the support of Russia and Iran and regards US forces as occupiers.