4 troops killed in southern Afghanistan insider attack The six casualties brought to 51 the number of coalition forces killed by their Afghan partners this year, most of them Americans. The insider attacks underscored the continued vulnerability of international troops despite intense efforts by U.S. and Afghan leaders to stem the killings. A senior official in Zabul province said a group of men wearing the uniforms of the Afghan National Police - a component of the country's 352,000-member security forces -- were involved in the pre-dawn Sunday incident in the Mizan district, which also wounded several other international troops. The official, who spoke on condition of anonymity because of the sensitivity of the matter, said he believed that the four victims were American troops. The NATO-backed International Security Assistance Force did not provide nationalities of the dead and injured. In a statement on its web site Sunday, the Taliban said four Americans died, crediting "one Afghan Mujahid," or holy warrior, for carrying out the attack. The spate of attacks this year have increasingly sown mistrust and strained the partnership between Afghan and Western forces battling the Taliban insurgency. Afghan officials said they have undertaken a massive rescreening process to weed infiltrators and potential turncoats from the ranks, but the attacks threaten to undermine the U.S. exit plan from Afghanistan that relies on Afghan forces taking responsibility for the country's security by the end of 2014. Lt. Col. Hagen Messer, a coalition spokesman, said there was no evidence to suggest that the two insider attacks in two days had a connection to the controversial anti-Muslim film that has provoked protests and violence in some 20 countries. "I cannot deny or confirm that these incidents are related to the film," Messer said. Every one of these insider attacks is handled as its own individual incident. We can't draw the conclusion that there is a single reason or point. Since 2007, when the insider- attack phenomenon began, about 109 international troops have been killed by rogue Afghan security forces. Last week, the Taliban called for increased attacks in Afghanistan, specifically on U.S. forces, to avenge the anti-Muslim film, as well as "all the violations against our heavenly book [the Koran]" and the Prophet Muhammad. "The Mujahideen in Afghanistan should avenge these actions of the American government by dealing a heavy blow to its invading troops on the battlefield," an official Taliban web site said. Local media reported that the head of the Zabul provincial council, Abdul Bari Barakzai, said six Afghan National Police were involved in Sunday's attack, and that five of them fled after killing four U.S. soldiers. The other was shot dead. Helman province's deputy police chief, Ismail Hotak, identified the troops killed on Saturday as British and said four other troops were wounded. The perpetrator was believed to be a member of the 16,000-member Afghan Local Police, the smallest component of the Afghan national forces. In Kabul, hundreds of students and others marched peacefully Sunday morning to protest the film, with some chanting "Death to America" and "Death to Israel." Javed Hamdard and Syed Salhuddin contributed to this report.