NATO forces in Afghanistan are reporting that three coalition helicopters crashed in two unrelated incidents Monday, killing at least 11 U.S. troops and three American civilians.

A NATO statement said one helicopter crashed in western Afghanistan after U.S. and Afghan troops raided a militant hideout and killed 12 suspected enemy fighters.

The statement said seven U.S. troops and three U.S. civilians were killed.  The injured include 14 Afghan troops, 11 American troops and one U.S. civilian.

The cause of the crash has not been determined, but NATO officials said they do not believe the helicopter was shot down by insurgents.

In another incident in southern Afghanistan early Monday, two helicopters collided and crashed, killing four U.S. service members.

A NATO statement said two other service members were injured in the incident.  NATO said officials are continuing to investigate the cause of the collision, but they have determined that hostile fire was not involved.

This past year has been the deadliest for foreign troops in Afghanistan since the U.S.-led invasion in late 2001 to fight the Taliban and al-Qaida militants.