The UN says up to eight houses were totally destroyed
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.
The UN investigation was carried out by its Assistance Mission in Afghanistan (Unama).
Such attacks are undermining attempts to win over Afghans
"Investigations by Unama found convincing evidence, based on the testimony of eyewitnesses, and others, that some 90 civilians were killed, including 60 children, 15 women and 15 men," the UN's Special Envoy to Afghanistan, Kai Eide, said in a statement issued on Tuesday.
"The destruction from aerial bombardment was clearly evident with some seven-eight houses having been totally destroyed and serious damage to many others.
"Local residents were able to confirm the number of casualties, including names, age and gender of the victims."
Call for help
The UN investigation adds further weight to the Afghan government's own report into the incident. President Karzai's office said on Sunday that "89 of our innocent countrymen, including women and children" died in the incident.
Reports say that Afghan soldiers were fighting Taleban militants in the village of Azizabad and called in US air forces for help.
President Karzai subsequently sacked Gen Jalandar Shah Behnam, head of the army in western Afghanistan, and Maj Abdul Jabar, for "neglecting their duties and concealing the facts", indicating that they were partly to blame for the incident.
The US has said it is carrying out its own investigation into the attack.
An Afghan general said the air strike was launched following intelligence that a Taleban commander, Mullah Siddiq, was presiding over a meeting of militants.
Afghan tribal elders said a bomb was dropped on a large group of mourners at a funeral wake.
The issue of civilian casualties has been a constant source of friction between Mr Karzai and international forces.