KABUL: Firing from NATO helicopters in an eastern province Friday killed six Afghan militiamen who allegedly shot at the aircraft, an Afghan official said. The incident in Khost province sparked angry protests. Hundreds of villagers shouting slogans against America and carried the bodies to the provincial governor's home to protest, Youqib Khan, the deputy provincial police chief, told The Associated Press. “What I confirm now is six local security forces were killed by NATO's helicopters. NATO told us the militiamen fired first. We don't know who to blame,” Khan said. The militiamen, part of a community security force, were not affiliated with the Afghan government. The attack came in a mountainous region of Khost, though it was unclear exactly where, he said. NATO said it was checking on the shooting account. It comes after NATO helicopter strikes on Pakistani territory killed two Pakistan border guards Sept. 30. In apparent retaliation, Pakistan shut a key border crossing that is the main route used by NATO and the U.S to bring supplies to forces in Afghanistan. The US apologized, but Pakistan has yet to reopen the crossing. The closure has stranded hundreds of trucks in areas where they are exposed to militant attacks. NATO said Friday a senior Taliban leader accused of commanding an “assassination cell” in southern Kandhar city was captured Thursday. The unidentified insurgent may be linked to the killing of Kandahar's deputy mayor, Noor Ahman, on Monday, as well as an Afghan military officer, the alliance said in a statement. Another Taliban commander was detained the same day in Kandahar province's Zhari district after a gunbattle left one insurgent dead, NATO said. Violence has escalated in southern Afghanistan since NATO forces launched operation Dragon Strike last month in areas around Kandahar city – 260 miles (420 kilometers) southwest of the capital Kabul – to flush out militants and destroy their strongholds. Southern Afghanistan is the birthplace of the Taliban, and the hard-line militia's presence is strong there. Meanwhile, four militants were killed and one captured by an Afghan-NATO force in eastern Wardak province's Chaki Wardak district Thursday, said NATO. The force took small-arms fire from Taliban fighters in several compounds, and retaliatory fire killed the four. Thursday was the ninth anniversary of the American invasion of Afghanistan. This week also marked another milestone as the death toll for NATO forces surpassed 2,000. At least 2,004 NATO service members have died fighting in Afghanistan since Oct. 7, 2001, according to an AP count.