The US-led NATO force in Afghanistan on Wednesday conceded that eight young men were killed during a recent air strike but insisted they were armed teenagers who posed a "threat" to soldiers.

Afghan President Hamid Karzai condemned the air strikes in the northeastern province of Kapisa, where French troops are based, and ordered an investigation after saying that eight children were killed on February 8.

British Air Commodore Mike Wigston told a news conference that there was no doubt that eight young men were killed in the bombing raid.

He expressed his condolences over the deaths, saying they were not targeted because they were necessarily Taliban or insurgents, but because they appeared a threat to Afghan and French troops operating in the area.

"I'm in no doubt of what villagers said, that eight young men died on that day. It's clear to me that it's the case. Who they were, what they were doing, we may never know for certain," Wigston told reporters.

Mohammad Tahir Safi, a member of parliament for Kapisa and part of an investigation team dispatched by Karzai, said the dead were aged between six and 14, with another man aged 18 to 20 who was mentally ill.

NATO said the military was examining photographs of the bodies to estimate their ages, but said they were "closer to 15-16, with one older".

"These were young Afghans. They were adult sized, athletic, strong, walking perfectly in the valley. I have no doubt that they were carrying weapons," Wigston told reporters.

It was "not the first time that children have been seen carrying weapons" and it was "not unusual for young males of this age to hold weapons".

He described the area in Kapisa as "dangerous" and a place of "enmity between the villages".

He disputed an account from Afghan investigators saying that local Afghan security forces did not back the operation, saying "there's no doubt that there was full cooperation of the Kapisa police."

"They were not bombed because we thought they were Taliban. They were not bombed because we thought they were insurgents or smugglers. We bombed them because we thought they were a threat.

"Had they gone over the hill to another valley, they wouldn't have been a threat and they wouldn't have been bombed. But they were in a position in which they could really easily threaten. It's a huge regret," Wigston said.

Safi said the children had gathered to start a fire about 600 metres west of a village where troops were operating when "all of a sudden a plane dropped one bomb in first round and another bomb later".

At the time, Kapisa district police chief Abdul Hamid Erkin told AFP that seven children and a mentally-handicapped 20-year-old were killed.

He said French commanders "claimed that the target was a group of Taliban facilitators, but we checked the area and there were no Taliban".

Last month, French President Nicolas Sarkozy said troops would hand over responsibility for security in Kapisa to Afghans from March 2012, following the killing of four French soldiers by an Afghan they were training.