President Karzai’s security chiefs have demanded that America should hand over the gunmen behind a night raid in eastern Afghanistan that government investigators and the United Nations say killed at least eight schoolchildren.

It was the Afghan Government’s most aggressive response yet to an alleged attack on civilians. But the US insisted that its men had come under fire and that all the victims were part of an Afghan cell manufacturing bombs.

The call heightens a war of words between the Afghan Government and its powerful military backers. It is the first time that Mr Karzai has tried to hold foreign forces directly accountable for killing civilians, although he has issued impassioned responses to civilian casualties that threaten to undermine Nato’s mission in Afghanistan.

It also reflects the growing assertiveness of a Government that precariously held its position after fraudriddled elections in August and open criticism from Nato countries over corruption.

Kai Eide, the head of the UN in Afghanistan, issued a statement reinforcing Afghan claims that most of the dead were schoolboys. “Based on our initial investigation, eight of those killed were students enrolled in local schools,” he said.

He accepted that there was evidence to suggest that insurgents were in the area, but reiterated concerns that night raids by US special forces risked undermining consent for foreign forces in Afghanistan.

“The United Nations remains concerned about night-time raids given that they often result in lethal outcomes for civilians,” he said.

The National Security Council, chaired by Mr Karzai, accepted the findings of an Afghan investigation that contradicted Nato’s claims. It demanded: “Those responsible for the deaths of those innocent youths must be handed over to the Afghan Government”.

Conventional US units told investigators that they had no knowledge of the operation, in Narang district in eastern Kunar province. Assadullah Wafa, who led the investigation, said that US troops flew to Kunar from Kabul late on Saturday. Nato sources said that the foreigners involved were non-military, suggesting that they were part of a secret paramilitary unit based in the capital.

Mr Wafa said that they landed helicopters outside the village and walked in at the dead of night before shooting the children at close range. “They were children, they were civilians, they were innocent,” he said. “I condemn this attack.”

The Security Council endorsed his findings. “International forces entered the area and killed ten youths, eight of them school students inside two rooms in a house, without encountering any armed resistance,” a statement from Mr Karzai’s office said.

A Western official was also scathing. “There’s no doubt that there were insurgents there, and there may well have been an insurgent leader in the house, but that doesn’t justify executing eight children who were all enrolled in local schools,” he said. Rahman Jan Ehsas, the local headmaster, told The Times that seven of the students were handcuffed before they were shot. A local farm labourer and a shepherd boy were also killed, he said.

The deaths sparked protests across Afghanistan, with students in Jalalabad burning an effigy of Barack Obama and children in Kabul as young as 10 demanding that foreign forces should quit Afghanistan.

Nato’s International Security Assistance Force said there was “no direct evidence to substantiate” the Government’s claims that unarmed civilians were harmed in the “joint coalition and Afghan security force” operation. The National Security Council made no reference to any Afghan forces involved in the operation. In the past, Special Forces have been criticised for using private Afghan militias in operations.