KABUL (AFP) - An airstrike by international forces mistakenly killed nine Afghan soldiers on Wednesday, the Defence Ministry said, in the latest in a string of errors to hit the campaign against Taliban militants. Separately, 55 militants and three policemen were killed in fighting in the southern province of Uruzgan to repel an attack by dozens of rebels, the US military and Afghan police said. The US-backed Karzai government condemned the killing of the soldiers by foreign forces and said such incidents could sap the morale of security forces who are fighting a resurgent Taliban movement. The US military admitted that coalition forces "may have mistakenly killed and injured" Afghan soldiers in the eastern province of Khost and said it was investigating. Foreign helicopters accidentally targeted the soldiers as they manned a checkpost in an operation to provide security for voters registering for elections, said Col Muhammad Gul, a local spokesman for the Afghan army. "In an international military air strike at 2:00 am... nine members of the Afghan National Army (ANA) were martyred and three others were injured," a defence ministry statement said. A spokesman for the US-led coalition forces in Afghanistan, Colonel Greg Julian, announced a joint investigation. "Initial reports from troops on the ground indicate that this may be a case of mistaken identity on both sides," a statement from his office said. Afghanistan's Defence Ministry condemned the incident and pledged to bring the "culprits" to justice. "Such tragic and painful incidents would weaken the morale of the national security forces and could hamper the national army development," it said. Late Tuesday, about 100 militants attacked Dih Rahwud district centre in the southern province of Uruzgan sparking a battle that lasted into early Wednesday, police said. "Our police bravely resisted and killed 35 Taliban whose bodies are left in the area," provincial police chief Juma Gul Hemat told AFP. "Three of our policemen were also martyred and nine others were injured." The US military later issued a statement saying 55 militants were killed in the battle. Troops also killed and wounded dozens of "enemies of the people" Tuesday in the southern province of Helmand, the Afghan defence ministry said. And in the western province of Herat, five police officers were captured by Taliban in an overnight raid on their post, a local official said. A local Taliban commander claimed the men had surrendered to the rebels but this could not be independently verified.