WASHINGTON – Twenty-two US service members were injured in a helicopter “mishap” in northeast Syria on Sunday, according to the US military, which did not specify the reason of the incident or the severity of the injuries.

The US Central Command said that ten service men were relocated to higher-level care institutions outside the region.

The Central Command, which controls US soldiers in the Middle East, stated that no enemy fire was detected, but that the reason of the incident was being investigated.

US Central Command officials did not immediately respond to demands for additional information.

The US-backed Syrian Democratic Forces, which hold large swaths of northeast Syria, deferred queries to the US-led coalition, which has American forces stationed there.

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The autonomous Kurdish-led administration that rules the region, as well as the central Syrian government in Damascus, did not reply quickly to demands for comment.

Approximately 900 US forces are stationed in Syria, the most of whom are in the east, as part of a mission to combat Islamic State remnants. In recent years, Iran-backed militants have repeatedly attacked American troops there.

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

US military first entered Syria during the Obama administration’s anti-ISIS operation, collaborating with a Kurdish-led militia known as the Syrian Democratic military.

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While the Islamic State is now a shadow of the group that declared a caliphate over a third of Syria and Iraq in 2014, hundreds of fighters remain camped in desolate areas where neither the US-led coalition nor the Syrian army, with support from Russia and Iranian-backed militias, have complete control.

Thousands more Islamic State fighters are being held in detention camps secured by the Kurdish-led Syrian Democratic Forces, America’s main ally in Syria.

According to US officials, the Islamic State has the potential to re-emerge as a major danger.

Threats from Iran-backed militias against US forces highlight Syria’s complicated geopolitics, where Syrian President Bashar al-Assad leans on assistance from Iran and Russia and regards US troops as occupiers.