The report said most of the civilian victims were children
A report by the independent human rights commission in Afghanistan says 97 civilians were killed in a US air attack earlier this month.
The figure differs from that of the US military, which says it believes 20 to 30 civilians may have been among up to 90 people killed.
The deaths occurred in the province of Farah, during a battle between Afghan and US security forces, and insurgents.
President Karzai says civilian deaths are boosting support for insurgents.
The report by Afghanistan's Independent Human Rights Commission - AIHRC - is also very different from earlier Afghan government figures, which said up to 140 civilians died in the US air attack.
This independent commission says it believes that the vast majority of those killed in Bala Baluk district were not armed insurgents, but children.
After a week-long study, the report's initial conclusion is that 65 children and 21 women died in the US air attack, along with 11 adult male civilians.
The air strike took place after insurgents attacked Afghan police positions.
The commission says a group of up to 300 militants knowingly placed civilians at risk by sheltering in their houses.
But it accuses the US military of an excessive response.
It says: "AIHRC believes that the level of force used by pro-government forces, particularly in the follow-up air strikes, was disproportionate."
The Americans' own investigation concluded that US forces acted appropriately.
A spokesman told the BBC there had been little - if any - indication that civilians were present when the bombs were dropped.
He said insurgents had used civilians as human shields, fully aware of the consequences.
The United Nations calculates that more than 2,000 civilians died in fighting in Afghanistan last year.
Most were killed by insurgents.
The coalition forces have revised their rules of engagement, to try to minimise the loss of civilian life.
But the BBC's Sarah Rainsford, in Kabul, says every mistake plays into the hands of the Taliban.
She says mistakes fuel opposition to the troops' presence - and to the elected government of Afghanistan that they're in the country to support.