Twenty-two US soldiers were injured in a helicopter accident in northeastern Syria at the weekend that did not involve any reported enemy fire, US Central Command said Tuesday.

“A helicopter mishap in northeastern Syria resulted in injuries of various degrees to 22 US service members” on Sunday, Centcom said in a statement.

“The service members are receiving treatment for their injuries and 10 have been evacuated to higher care facilities,” it said.

“The cause of the incident is under investigation, although no enemy fire was reported.”

The United States has about 1,000 troops deployed in Syria as part of international efforts to combat jihadists, and periodically carries out strikes targeting militants in the country.

US bases have been repeatedly targeted in the past year, with cells linked to the Islamic State group claiming responsibility for some of them.

In March the United States carried out air strikes on Iran-linked groups in Syria after a US contractor was killed in a drone attack on a US-led coalition base near the city of Hasakeh in northeastern Syria.

At the time, the Syrian Observatory for Human Rights war monitor said at least 19 people were killed in the US air strikes.

The conflict in Syria has killed around half a million people since it began in 2011 with a brutal crackdown on anti-government protests, spiralling into a complex battlefield involving foreign armies, militias and jihadists.