All four NATO troops killed in a helicopter crash in southern Afghanistan on Thursday were American, US officials said.
The Blackhawk helicopter went down in stormy weather after responding to a suicide attack in Helmand province, Pentagon spokesman Captain John Kirby told reporters.
"It appears that the weather was the principle cause," Kirby said.
Helicopter crashes occur with some regularity in Afghanistan, but the NATO-led force says they are rarely the result of Taliban fire.
On March 16, 12 Turkish soldiers and two civilians were killed in a chopper crash in the Afghan capital Kabul.