KABUL, (Reuters) – Six U.S. Marines were killed in a helicopter crash in southern Afghanistan on Thursday, a U.S. official said, but NATO said Taliban fighters were not active in the area at the time.

“The cause of the crash is under investigation, however initial reporting indicates there was no enemy activity in the area at the time of the crash”, a spokesman for the NATO-led International Security Assistance Force said Friday, raising questions about a possible malfunction or pilot error.

The NATO spokesman declined to confirm the nationality of the victims but a U.S. official, speaking on condition of anonymity, said all of the dead were U.S. Marines.

It is the worst crash since August last year when 30 American forces, including 22 elite Navy SEAL commandos, died when their helicopter came down in eastern Afghanistan.

An investigation into the incident confirmed that the Taliban fired a rocket-propelled grenade that hit one of the rotary blades and exploded, sending the helicopter plunging to the ground and bursting into flames within seconds.

All eight Afghans on board were also killed.