US Secretary of State Hillary Clinton says Washington "deeply, deeply" regrets the deaths of Afghan civilians in an air strike in the Farah province.
Officials say about 30 of the casualties were Taliban fighters, but the rest were civilians who were taking shelter from the attack.
President Harmid Karzai of Afghanistan welcomed the apology and said it was important to continue the fight against terrorists.
Mrs Clinton says there will be a joint investigation into the deaths.
