A U.S. military official in Kabul confirmed the strike and said the international coalition was investigating the incident.

American war aircraft and remotely piloted drones still carry out strikes in Afghanistan, targeting Taliban and other militants even after most foreign forces withdrew at the end of last year.

Monday's early morning air strike was in Baraki Barak district of Logar province, just south of the capital Kabul, where the Taliban insurgents frequently launch assaults on Afghan forces.

"It seems the attack was carried out mistakenly on army forces, and an investigation is ongoing to find its reason," said General Abdul Raziq, a senior Afghan National Army commander in Logar.

A later statement by Afghanistan's Ministry of Defence said that helicopters belonging to the U.S.-led military coalition had come under enemy attack in the area and returned fire, mistakenly hitting the army post.