Kabul, Afghanistan (CNN) -- Four U.S. service members were killed when two helicopters were apparently involved in a mid-air collision in southern Afghanistan on Monday, NATO's International Security Assistance Force said.
Along with those killed, two other NATO-led service members were injured in the incident.
"The incident is currently being investigated, but it is confirmed that hostile fire was not involved," ISAF said in a news release.
There was also a separate incident on Monday involving a helicopter in western Afghanistan that resulted in an unknown number of reported military casualties.
ISAF said a joint, international security force killed "more than a dozen enemy fighters" as they searched a compound. The location was believed to harbor insurgents involved in activities related to narcotics trafficking in western Afghanistan.
The militants were killed in a firefight when insurgents confronted the joint force. As the force was leaving, a helicopter "went down due to unconfirmed reasons."
Along with reported military casualties, a recovery operation has been launched.