The US military said four troops were killed and two others injured in a mid-air collision of two helicopters in southern Afghanistan.
Hostile fire was ruled out in the crash, which occurred this morning. No further details are being released.
In an unrelated incident, another helicopter went down today during a joint, international security force operation against insurgents in western Afghanistan in which a dozen militants were killed.
The US said military casualties were reported and a recovery operation is under way.