A US military inquiry has found that an air strike on militants in western Afghanistan on 22 August killed many more civilians than first acknowledged, BBC reported.
US Central Command said 33 civilians, not seven, had died in the village of Azizabad in Herat province.
While voicing regret, it said US forces had followed rules of engagement.
Afghan officials and the United Nations feared at the time that up to 90 people had died in the strike on Azizabad, including 60 children.
Video footage, apparently of the aftermath of the raid, showed some 40 dead bodies lined up under sheets and blankets inside a mosque.
The majority of the dead captured on the video were children, babies and toddlers, some burned so badly they were barely recognisable.
US forces had originally said seven civilians were killed in a "successful" US raid targeting a Taleban commander in Azizabad.
Announcing the findings of US Central Command's inquiry, Lt Gen Martin Dempsey said that US forces had acted on credible intelligence, in self-defence and in line with rules of engagement.
The US forces had called for air support during the operation with Afghan troops, after they were fired upon from a suspected Taleban compound.
Lt Gen Dempsey said that 22 insurgents had also been killed in the attack.
"We are deeply saddened at the loss of innocent life in Azizabad," he added.
"We go to great lengths to avoid civilian casualties in Afghanistan in all our operations, but as we have seen all too often, this ruthless enemy routinely surround themselves with innocents."