At least 14 American soldiers have died after a series of air crashes in Afghanistan.

Seven US troops and three civilians were killed when a helicopter crashed in western Afghanistan. The cause of the crash has yet to be confirmed, although US military sources have ruled out hostile fire.

An apparent mid-air collision killed another four American soldiers in Afghanistan on Monday as President Hamid Karzai questioned the reliability of the United States ahead of a run-off election.

The collision in southern Afghanistan came as NATO-led forces reported that another helicopter crashed in the northwest of the country following a raid on suspected drug traffickers.

The current year has been the most deadly for the more than 100,000 NATO and US troops fighting a resurgent Taliban since the 2001 US-led invasion ousted the Islamists’ extremist regime in Kabul.

NATO’s International Security Assistance Force (ISAF) said the crash in the south was currently being investigated but no hostile fire was involved.

“Four ISAF service members were killed and two others injured in the incident,” it said in a statement, adding that the two helicopters had apparently collided in mid-air.

Colonel Wayne Shanks, US military spokesman in Kabul, confirmed the four dead soldiers were Americans.

The second crash happened in western Baghdis province during a joint search operation by Afghan and Western forces of “a suspected compound believed to harbour insurgents conducting activities related to narcotics trafficking”.

“We are still trying to get the wounded out, we are still getting numbers from the manifest, we do not know how many were aboard the helicopter. We are still trying to get the number of wounded,” said Captain Mike Andrews of ISAF.

Western Afghanistan has become increasingly hostile in recent months as the Taliban spreads its tentacles across the country.