In a statement yesterday, the alliance said the helicopter "crashed due to a non-hostile incident October 11 at approximately 4:15 pm at Camp Resolute Support, Kabul."
The alliance says the incident is under investigation. The nationalities of those killed and injured were not released, according to protocol.
Earlier, US Army Col Brian Tribus, the military spokesman, said an "incident" involving a NATO aircraft and an observational balloon had taken place "in the vicinity of the Resolute Support base."
An Afghan security guard said the military helicopter appeared to strike the balloon as it was landing.