Five members of a NATO-led advisory unit were killed Sunday in a helicopter crash near Kabul, Afghanistan, the U.S. military said in a press release. Five others were injured; their condition is not yet known.

According to the military, the helicopter "crashed due to a non-hostile incident,” at about 4:15 p.m. Officials have not yet released the names or nationalities of those killed or injured, but the BBC reported that two of those killed in the crash were personnel of the British Royal Air Force. 

The helicopter, a Puma Mk2, reportedly crashed as it made its descent into the base, a British Ministry of Defence spokeswoman told the BBC. "The incident is currently under investigation, but we can confirm that it was an accident and not the result of insurgent activity,” the spokeswoman said. Witnesses said the helicopter had struck a tethered surveillance balloon as it descended, the Telegraph reported.

The NATO-led group, Resolute Support, does not engage in combat missions. The 12,000-person unit was launched on January 1, 2015. Its mission, according to NATO's website, is “to provide further training, advice and assistance for the Afghan security forces and institutions.”