A helicopter accident in northeastern Syria over the weekend left 22 American service members injured, the U.S. military said Tuesday, adding that the cause of the accident was under investigation and that no enemy fire was involved.

The military statement said the service members were receiving treatment and 10 were moved to “higher care facilities” outside the region.

“A helicopter mishap in northeastern Syria resulted in the injuries of various degrees of 22 U.S. service members,” it said. “No enemy fire was reported.”

A spokesman for the U.S.-backed Syrian Kurdish forces did not immediately respond to an Associated Press request for comment.

There are at least 900 U.S. forces in Syria on average, along with an undisclosed number of contractors. U.S. special operations forces also move in and out of the country, but are usually in small teams and are not included in the official count.

U.S. forces have been in Syria since 2015 to advise and assist the Kurdish-led Syrian Democratic Forces in the fight against the militant Islamic State group. Since the extremist group was defeated in Syria in March 2019, U.S. troops have been trying to prevent any comeback by IS, which swept through Iraq and Syria in 2014, taking control of large swaths of territory.

However, IS sleeper cells remain a threat. There are also about 10,000 IS fighters held in detention facilities in Syria and tens of thousands of their family members living in two refugee camps in the country’s northeast.

Over the past years, U.S. troops have been subjected to attacks carried out by IS members and Iran-backed fighters there. In late March, a drone attack on a U.S. base killed a contractor and wounded five American troops and another contractor. In retaliation, U.S. fighter jets struck several locations around the eastern province of Deir el-Zour, which borders Iraq.

U.S. Defense Secretary Lloyd Austin said at the time that the strikes were a response to the drone attack as well as a series of recent attacks against U.S.-led coalition forces in Syria by groups affiliated with Iran’s Revolutionary Guard.

In a related development, Syrian Kurdish-led authorities announced Saturday that hundreds of IS fighters held in prisons around the region will be put on trial after their home countries refused to repatriate them.