Nato said the reason for the crash was "unconfirmed", ruling out hostile fire.

Mid-air collision

Another four US service members were killed and two others were wounded in an apparent mid-air collision between two helicopters in southern Afghanistan, Nato said in an earlier statement.

Colonel Wayne Shanks, from the US Army, said that investigations into both incidents were being hampered by "combat conditions".

"This is where we see the nexus between the insurgency and the narcotics trade," he said, adding that a dozen Taliban fighters were killed in the operation carried out by the helicopter brigade in the west of the country.

Nato and US troops are suffering the highest fatality rates since the deployment of foreign troops to Afghanistan in 2001.

So far in October, more than 30 US soldiers have died.

The deaths come as the US debates whether to send thousands more troops to the country and the Afghan government prepares for a runoff election between Hamid Karzai, the Afghan president, and his main challenger following an August vote that was discredited due to alleged widespread ballot-rigging.