Six members of the U.S.-led NATO International Security Assistance Force service were killed in an aircraft crash in southern Afghanistan Tuesday, officials said.

The cause of the crash is under investigation, NATO said in a statement, adding that “initial reporting indicates there was no enemy activity in the area at the time of the crash.”

NATO did not disclose the nationalities of the victims but A U.S. defense official said they were American.

The deputy governor of southern Zabul province, Mohammad Jan Rasoolyar, says a helicopter crashed in the remote district of Shajau, The Associated Press reported. It was not immediately clear if he was describing the same incident.