KABUL (Reuters) - The Nato-led force in Afghanistan said on Monday it was investigating another case in the east where its troops may have accidentally killed Afghan civilians, a day after local officials said 64 had been killed in raids there. The International Security Assistance Force (ISAF) said in a statement it was investigating an operation in eastern Nangarhar province on Sunday night "that resulted in Afghan civilians being accidentally killed and wounded". The statement did not give any details about the number of civilians involved. It said three insurgents had been fired on after they were seen placing a roadside bomb. Three vehicles were later seen driving to a nearby hospital and those inside said the roof of their compound had collapsed during the engagement, ISAF said. "This is a deeply regrettable accident," ISAF spokesman US Army Colonel Patrick Hynes said in the statement. Civilian casualties in NATO-led military operations, often caused by air strikes and night raids, have long been a source of friction between the Afghan government and its Western backers. Late on Sunday, Afghan President Hamid Karzai strongly condemned raids in eastern Kunar province which his office said killed more than 50 civilians. Kunar Governor Fazlullah Wahidi earlier told Reuters that 64 civilians were killed by ground and air strikes in the Ghazi Abad district during operations spanning four days. Videos taken by Reuters television in a hospital in the provincial capital, Asadabad, showed two children being treated for leg wounds alongside two wounded women. Two other witnesses at the hospital said between 52 and 62 civilians had been killed. ISAF however cast doubt on the toll in Kunar but said an investigation with Afghan officials would begin on Monday. Rear Admiral Greg Smith, the chief ISAF spokesman, told Reuters the investigation would centre on a firefight that began in a rugged and remote area on Thursday night and lasted more than five hours. He said ISAF had "clear intelligence" that Taliban leaders were planning a meeting that evening and that surveillance footage from weapons systems did not indicate any civilians, or permanent settlements, were in the area. Rules governing air strikes and night raids have been tightened significantly by NATO-led forces in the past two years, leading to a drop in civilian casualties. A United Nations report late last year found that civilian casualties in Afghanistan rose 20 percent in the first 10 months of 2010 compared with 2009, with more than three-quarters killed or wounded by insurgents. The report found there were 6,215 civilian casualties in the period, including 2,412 deaths. Those caused by Afghan and foreign "pro-government" forces accounted for 12 percent of the total, an 18 percent drop.