A U.S. investigation into an air strike in western Afghanistan last year was "deeply flawed" and casts doubt over the military's commitment to reduce civilian casualties, a human rights group said on Thursday. The accidental killing of Afghan civilians has eroded support for international forces in the country and opened a rift between President Hamid Karzai and his Western backers. After a U.S. air strike in Azizabad in August, U.S. forces reported five to seven civilian deaths. Both the Afghan government and the United Nations said more than 90 civilians had been killed. The U.S. military reopened its investigation after a video emerged showing bodies of victims. The second inquiry, led by U.S. Air Force Brigadier General Michael Callan, found 33 civilians had been killed. But Human Rights Watch (HRW) criticized the way it was conducted and questioned the U.S. army's pledge to protect civilians in military operations. "The U.S. military's investigation into deadly and controversial air strikes in Azizabad in Afghanistan in August 2008 was deeply flawed," the New York-based HRW said. "The weaknesses in the Callan Report Summary call into question the depth of the Defense Department's commitment to institute reforms that would reduce civilian casualties," it said in a letter to U.S. Defense Secretary Robert Gates on Thursday. The U.S. military, HRW said, failed to admit making any mistakes in its initial assessments and "exonerated" U.S. forces of any wrongdoing. The U.S. military also dismissed claims by Afghan villagers that some of the graves contained more than one body and assumed almost all the men who died were insurgents, HRW said. The U.S. commander of NATO-led forces in Afghanistan tightened procedures following the incident and issued a directive aimed at reducing civilian deaths. But with 30,000 new troops due in Afghanistan this year, civilian deaths could increase if U.S. procedures do not change further still, HRW said.