Sixty children were killed in air strikes by US-led coalition warplanes in western Afghanistan last week, a UN investigation has found. UN investigators said they discovered "convincing evidence" that a total of 90 Afghan civilians died in the incident.

The toll, potentially the worst since the fall of the Taliban in 2001, could wreck relations between the Afghan government and the Nato-led coalition forces, which were already under severe strain over civilian casualties and strategy in the counter-insurgency against the Taliban.

The government of President Hamid Karzai has ordered that any military operation by foreign forces on its territory will be subject to a new set of rules enforceable under international law.

Kai Eide, the UN special envoy to Afghanistan who ordered the investigation, said the incident could undermine the faith of the Afghan people in international efforts to stabilise the country.

Military sources said the air strikes last Thursday on the Shindand district of Herat province were carried out not by the Nato force attempting to bolster Karzai's government, but as part of a parallel US mission targeting al-Qaida and Taliban militants, called Operation Enduring Freedom (OEF).

US officials initially said that the air strikes were aimed at a Taliban stronghold and had killed 30 jihadis. An OEF spokesman in Kabul said last night that an investigation into the incident had been launched last Saturday and was still under way.

In his report, Eide said investigators from the UN Assistance Mission in Afghanistan (Unama) found that up to eight houses in the village of Nawabad had been destroyed in the raids and many others damaged.

"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. Fifteen other villagers were wounded or otherwise injured," Eide wrote.

"This is matter of grave concern to the United Nations, I have repeatedly made clear that the safety and welfare of civilians must be considered above all else during the planning and conduct of all military operations. The impact of such operations undermines the trust and confidence of the Afghan people in efforts to build a just, peaceful, and law-abiding state."

Humayun Hamidzada, a spokesman for the Afghan president, said Karzai had ordered that all foreign military operations be governed by an internationally enforceable "status of forces agreement".

"The patience of the Afghan people has run out. We no longer can afford to see the killing of our children," Hamidzada said.

The incident comes at a fraught time for western forces in Afghanistan, after a week of high casualties and deep splits within Nato on sharing the burden of the Afghan conflict.

Eide was appointed to bring some coordination to the international community's disparate efforts. But last night he warned that those efforts were in danger of being crippled by public mistrust.

In a harshly worded statement, he said: "I want to remind all parties engaged in the conflict that the protection of civilians must be their primary concern; they must respect their duties under international humanitarian and human rights law to protect the people we are here to serve."