Six US service members were killed in southern Afghanistan when a helicopter crashed after developing engine trouble, officials said.
One person on board the aircraft survived, US defence chiefs added.
A statement issued by the Nato International military coalition said the crash is under investigation and that there was no insurgent activity in the area at the time.
A US official said investigators are looking into the cause of "the engine failure''.
The deputy governor of southern Zabul province, Mohammad Jan Rasoolyar, said a Nato helicopter crashed in the remote district of Shajau.
US officials later confirmed that Zabul was the location of the US crash.
This year, 109 members of the US military have died in Afghanistan, out of a total of 139 members of the coalition.
The death toll has dropped significantly since the coalition handed over responsibility for security to Afghan forces last summer and coalition troops are now training and assisting.
By comparison, 394 foreign troops died last year, including 297 Americans.