The US military said a helicopter accident in northeast Syria has left 22 US service members injured, though no enemy fire was reported at the time of the “mishap”.

“The service members are receiving treatment for their injuries and 10 have been evacuated to higher care facilities outside of the CENTCOM AOR [US Central Command Area of Responsibility]”, the US military said in a statement on Monday.

An investigation is under way to determine the cause of the accident on Sunday in northeast Syria, US Central Command said in a statement.

The statement added that “no enemy fire was reported” at the time of the accident.

US forces operating in Syria since 2015 have come under sporadic attacks from ISIL (ISIS) fighters and Iranian-backed groups operating in the country.

At least 900 US soldiers are deployed in Syria, along with an undisclosed number of US military contractors, as part of operations to assist the Kurdish-led Syrian Democratic Forces in the fight against ISIL (ISIS).

In March, the governments of Iran and Syria condemned US forces for carrying out attacks on Syrian soil that reportedly killed 19 people, and were launched in apparent revenge for a drone attack that injured US soldiers and killed a US contractor.

The suspected Iranian-made drone killed one US contractor, injured another and wounded five US soldiers at their base in the northeast of the country near the city of Hassakeh.

The US retaliatory attacks were reportedly conducted by F-15 fighter jets that struck three locations, all in the vicinity of Deir Az Zor, a region in eastern Syria which borders Syria.

In April, US forces said they had killed a senior ISIL (ISIS) figure and two other members of the group in a “helicopter raid” operation in Syria.