Afghan President Hamid Karzai said on Wednesday an air strike by coalition forces earlier this week killed some 40 civilians and wounded about 28 in Kandahar province, Reuters reported.
Karzai called on U.S. President-elect Barack Obama to make it his priority to stop the killing of civilians.
Scores of Afghans have been killed in U.S. air strikes this year, leading to resentment against the presence of foreign troops and a rift between Karzai and his Western backers.
The air strike took place on Monday in the Shah Wali Kot district in the southern Taliban heartland of Kandahar province.
"By bombing Afghanistan, the war against terrorism cannot be won," Karzai told a news conference.
Several villagers who had taken a group of wounded to the hospital in Kandahar city said the air strike hit a wedding party.
The U.S. military said it was checking reports.
"The coalition and Afghan authorities are investigating reports of non-combatant casualties in the village of Wech Baghtu," said U.S. forces spokesman Commander Jeff Bender.
"If innocent people were killed in this operation, we apologize and express our condolences to the families and the people of Afghanistan," he said in a statement.
Karzai requested Obama make it his priority to end civilian casualties.
"The main problem, which has become a matter of tension (with the United States), is civilian casualties," Karzai said. "Civilian casualties should completely stop. The war in the villages of Afghanistan will never give fruit."
Some 4,000 people, around a third of them civilians, have been killed this year in fighting with the Taliban, who have expanded the scope and scale of their insurgency trying to oust Karzai's Western-backed government and eject foreign forces.
NATO and U.S.-led coalition troops in Afghanistan say they do their utmost to avoid civilian casualties, but mistakes happen. Far more civilians are killed in Taliban attacks, especially by suicide and roadside bombs.