**Four American soldiers were killed when two helicopters collided mid-air in southern Afghanistan, Nato-led forces said in a statement.**The incident is being investigated, but it is confirmed that hostile fire was not involved, the statement said.
Two soldiers were wounded in the crash which took place on Monday morning. They have been taken to hospital.
Meanwhile, another helicopter went down during an operation by international forces against insurgents in the west.
The statement said a dozen militants were killed as international troops searched a compound believed to harbour insurgents involved in drugs trafficking.
As the international force left the area, one of its helicopters went down and military casualties were reported, the statement added.
This year has seen the highest death toll of international troops in Afghanistan since the Taliban were overthrown in 2001.
There have been dozens of American soldiers among those killed.