There is convincing evidence that 60 children and 30 adults were killed in a US air strike in western Afghanistan last Friday, the United Nations says.
The US originally said its planes had killed 30 militants in the attack in the province of Herat.
President Karzai sacked two senior Afghan army officers over the incident. The US says Afghan forces led the operation in the district of Shindand.
The incident has worsened relations between Mr Karzai and foreign forces.
On Monday the government said it wanted to renegotiate the terms under which US-led forces and Nato-led forces operate in Afghanistan.
If confirmed, the Shindand incident is one of the worst cases of foreign forces killing Afghan civilians.