Four American troops were killed and two injured Monday when two helicopters collided in midair in southern Afghanistan. Hostile fire has been ruled out in the crash, a military statement said. U.S. military spokesman Col. Wayne Shanks said he did not have other details yet about the cause of the collision. Shanks said the injured had been evacuated to hospitals inside Afghanistan. In an unrelated incident, another helicopter went down Monday during an operation by international forces against insurgents in western Afghanistan in which a dozen militants were killed. The U.S. said military casualties were reported and a recovery operation is under way. Casualties could refer to either dead or injured. This has been the deadliest year for international and U.S. forces since the 2001 invasion to oust the Taliban. Fighting spiked around the presidential vote in August, and 51 U.S. soldiers died that month -- the deadliest for American forces in the eight-year war. So far in October, more than 30 American troops have died.