NATO said Sunday that its forces accidentally killed at least four civilians in eastern Afghanistan, while an official in the nation's west said foreign troops used air strikes against Afghan police, killing nine. The reported civilian and police deaths could damage popular support for the Afghan government as well as for foreign forces operating here. President Hamid Karzai has pleaded with the US and other nations fighting resurgent militants to avoid civilian casualties. NATO's International Security Assistance Force said it was investigating whether three other civilians also were killed Saturday night in the Barmal district of Paktika province when its troops fired two mortar rounds that landed nearly half a mile (1 kilometer) short of their target. The alliance said it was providing medical aid to four civilians who were wounded. "ISAF deeply regrets this accident, and an investigation as to the exact circumstances of this tragic event is now under way," it said in a statement. On the other side of the war-torn country in Farah province, a convoy of foreign forces showed up in Anar Derah district near the Iranian border and clashed with Afghan police, killing nine of them, said provincial Deputy Governor Younus Rasuli. He said the foreign troops had not informed local officials they were coming, and the police thought they were enemy fighters. The two sides fought from about midnight until 4 a.m. Sunday, and the foreign forces used airstrikes, Rasuli said. Both NATO and US coalition officials said they were looking into the report. Separately, a mine exploded under a civilian vehicle in Gereshk district in the southern province of Helmand on Sunday, killing three children and wounding four other people, said provincial police Chief Mohammad Hussein Andiwal. Andiwal accused Taliban militants of planting the mine on a road frequently used by Afghan and foreign troops. On Saturday, militants attacked a police checkpoint in the same district but in the ensuing gunfight three Taliban fighters were killed, Andiwal said. No police were injured in that clash, he said. Meanwhile, the Ministry of Defense said Afghan soldiers battled insurgents in southern Zabul province on Saturday, killing nine militants and wounding seven others. Afghanistan faces intensifying militancy nearly seven years after the US-led invasion of the country ousted the hard-line Islamic Taliban movement from power. More than 2,500 people have died in insurgency-related violence this year in the country, according to an Associated Press tally of official figures. Most have been militants, but the total includes hundreds of ordinary citizens. NATO and the coalition insist they take great precautions to avoid civilian casualties. Commanders accuse insurgents of endangering innocents by launching attacks from residential areas and by carrying out suicide attacks that kill far more bystanders than security personnel.