Four US soldiers were killed on Monday when two helicopters flying for the NATO-led International Security Assistance Force (ISAF) in Afghanistan were involved in what was believed to be a mid-air collision, the ISAF and US military said.
Hostile fire was not involved in the accident, the ISAF said.
Two soldiers were also injured in the accident in southern Afghanistan, it said, without specifying the location.
US Colonel Wayne Shanks confirmed the four fatalities were American troops.
A third helicopter also went down in western Afghanistan after a raid on a compound where suspected militants involved in the narcotics trade were located, the ISAF said.
Casualties were reported in the third crash, as rescue efforts were under way.
More than a dozen militants were also killed in a firefight that broke out with the international security force conducted the raid, the ISAF said.