Twenty-two U.S. service members were injured on Sunday in northeastern Syria in a "mishap" involving a helicopter, military officials said Monday.

"The cause of the incident is under investigation, although no enemy fire was reported," U.S. Central Command said in a statement.

The service members had "various degrees" of injuries, officials said. Ten were evacuated to health care facilities outside the region, the statement said.

Military officials in April ordered a 24-hour stand-down of all aviation units following two deadly mid-air helicopter collisions within 30 days.

Two Apache helicopters crashed in April in Alaska, killing three soldiers and leaving another injured.

Elsewhere, two HH-60 Blackhawk helicopters collided in late March near Fort Campbell, Kentucky, killing nine Army soldiers aboard.

“The safety of our aviators is our top priority, and this stand-down is an important step to make certain we are doing everything possible to prevent accidents and protect our personnel,” Army Chief of Staff Gen. James McConville said in a statement announcing the stand-down.

ABC News' Luis Martinez contributed to this story.