( CNN ) -- Afghan civilians were killed in U.S. airstrikes during fighting this week in western Afghanistan , local officials and the Red Cross said . Villagers pray at a mass grave this week after an airstrike in Afghanistan 's Farah province . The reports come as concerns mount over noncombatant casualties in the war against the Taliban . But the top U.S. commander in Afghanistan said the U.S. military suspects the incident started when Taliban militants entered the area and beheaded three civilians . And another senior U.S. military official said Taliban militants may have killed as many as 15 civilians with grenades and then paraded their shrapnel-riddled bodies through villages in western Afghanistan . U.S. Secretary of State Hillary Clinton said Wednesday she `` deeply regrets '' the loss of civilian lives but pointed out `` we do n't know all of the circumstances '' and promised an investigation . Afghan officials have said that a number of civilians died Tuesday during U.S. airstrikes on villages in Farah Province . The International Committee of the Red Cross said in a news release that its `` impression was that dozens of people , including women and children , had been killed . '' The ICRC workers were told that some of the dead had already been buried by the time they arrived on the scene . `` We are deeply concerned by these events , '' Reto Stocker , the ICRC 's head of delegation in Kabul , Afghanistan , said in a statement . `` We know that those killed included an Afghan Red Crescent volunteer and 13 members of his family who had been sheltering from fighting in a house that was bombed in an airstrike . '' But U.S. officials believe the Taliban deliberately engineered a ground attack against Afghan and U.S. forces , expecting the United States would call in airstrikes . They said the Taliban were then prepared to kill the civilians . A senior U.S. military official said there was `` very reliable intelligence '' that Taliban fighters rounded up three families , including women and children , and killed them with grenades . The official would not allow his name to be used because a preliminary investigation into the matter is ongoing and no conclusions have been reached . Their bodies , shrapnel wounds visible , were then put into the backs of trucks and driven through the area in an effort to convince villagers that the U.S. military operation had killed them . The official said he did not know who drove the trucks -- other Taliban or local Afghans forced into duty . `` No one is disputing people died , it 's how they died , '' the official said . `` What we do have is strong evidence to support that a number of women and children were killed by the Taliban and their bodies were driven by locals as evidence of U.S. bombing , '' the official said . Some Afghan civilians may have been killed or wounded before the airstrikes during ground fighting between the Taliban and Afghanistan and U.S. forces , the official said . But the military 's investigation found no evidence of large-scale civilian deaths resulting from the airstrikes themselves , despite the claims from Afghan officials and the Red Cross . U.S. military investigators found no evidence of human remains in any of the four bomb craters it examined , the official said . The news comes as Afghan President Hamid Karzai visits Washington this week . Karzai and his Pakistani counterpart , President Asif Ali Zardari , are talking with key congressional leaders and policymakers ahead of meetings with President Obama and Clinton . Speaking in Washington , Clinton -- flanked by Karzai and Zardari -- said the United States and Afghanistan will jointly investigate the airstrike and its aftermath . She said Washington already has sent a team of investigators to the region . The ICRC team found dozens of dead bodies in two villages , spokeswoman Jessica Barry in Kabul , told CNN . The increase of international troops in Afghanistan has raised a concern that fighting will intensify and civilians will face greater risks of getting hurt , she said . `` It 's all sides who need to respect the civilians , '' Barry said . `` And this is the message we pass to all the armed forces , to the international forces , to the Taliban , to the armed opposition . '' CNN 's Barbara Starr and Chris Lawrence contributed to this report .
NEW : Red Cross used grenades , paraded bodies through villages , official says . NEW : U.S. believes Taliban planned attack expecting Afghanistan to call in airstrikes . Civilians killed in U.S. airstrikes in U.S. , Taliban says . News of strike comes as ICRC president visiting Washington for talks .