Four US servicemen were killed in Afghanistan on Monday when two helicopters operated by NATO-led troops collided in mid-air, coalition forces said.
The NATO-led International Security Assistance Force (ISAF) said the incident occurred in southern Afghanistan. Two people were injured in the incident.
"The incident is currently being investigated, but it is confirmed that hostile fire was not involved," coalition forces said in a statement.
This year has seen a surge of violence in Afghanistan as an increasingly fierce Taliban step up operations against US and NATO forces operating in the country.
Separately, another helicopter went down in western Afghanistan during an operation against insurgents, NATO-led troops said, adding that more than a dozen Taliban fighters were killed in an ensuing firefight.
"As the joint force was departing the area, one helicopter went down due to unconfirmed reasons," it said. "Military casualties are reported and a recovery operation is underway."
An ISAF spokeswoman could not give any further information on the casualties.