US President George W Bush offered his condolences for the killing of at least 40 civilians, including a wedding party, by US planes in Afghanistan.
"The president is concerned about reports from Afghanistan that innocent lives were lost in the conduct of joint US-Afghan military operations in Uruzgan province," White House spokesman Mr Ari Fleischer said in a statement. "We do not know what happened and we are working very hard to find out."
Pentagon officials have refused to accept blame, saying their aircraft believed they were under attack. But President Bush extended his sympathies regardless of where the blame lay.
"On behalf of the American people, the president extends his deep condolences for the loss of innocent life no matter what the cause is determined to be," the White House said. "In the meantime, we are consulting with Afghan authorities on the humanitarian needs of the people in that area."
An Afghan and US team arrived at the site to investigate the reports, which came as American forces scoured the mountainous province of Uruzgan for Taliban leader Mullah Mohammad Omar, who was born close to the bombed village.
Afghan officials said wedding guests near the village of Deh Rawud were firing into the air - a tradition in Pashtun weddings - when they were mistakenly bombed by US forces.
"The number is some 40 people killed, all civilians, and some 100 people wounded," Foreign Minister Mr Abdullah Abdullah told a news conference. "In one village, there was a wedding party ... a whole family of 25 people. No single person was left alive. This is the extent of the damage."
US Defense Secretary Mr Donald Rumsfeld told reporters at the Pentagon the investigation could take "another day or two to come up with facts that could be useful".
"There cannot be the use of that kind of firepower and not have mistakes and errant weapons exist," he said. "It's going to happen. It always has and I'm afraid it always will."