The International Security Assistance Force (ISAF), to which Britain contributes 5,000 troops in southern Afghanistan, revealed that soldiers defused an explosive vest which had been placed on a six-year-old who had been told to attack Afghan army forces in the east of the country.

The boy was spotted after appearing confused at a checkpoint. The vest was defused and no one was hurt.

The claim came only hours after the second report this week that civilians had been killed in Nato military operations.

Nine women, three babies and the mullah of a local mosque died alongside 20 suspected Taliban militants after an air strike, Helmand's police chief, Mohammad Hussein Andiwal, said.

The air strike had been launched in response to an attack on Nato troops by militants near the town of Gereshk. An estimated 120 people have been killed in recent weeks, including seven schoolboys who died in a US air strike on Sunday.

Yesterday ISAF said it was investigating reports from the Afghan authorities that civilians had been killed. But it also accused the Taliban of using civilians as battleground cover, and said the incident with the boy signalled a new type of tactic. The boy had been ordered to target a check point in Miri, in the Andar district of Ghazni province.

"They placed explosives on a six-year-old boy and told him to walk up to the Afghan police or army and push the button," said Captain Michael Cormier, the company commander who intercepted the child, in a statement. "Fortunately, the boy did not understand and asked patrolling officers why he had this vest on."

Lieutenant Colonel David Accetta, ISAF eastern regional command spokesman, told the Guardian: "In the past we have not seen the Taliban sink that low, to use children as suicide bombers. The personnel secured the vest to make sure the child was safe."

Lt Col Accetta said the procedure for dealing with an armed minor had so far been untested in Afghanistan.

"It would have been difficult to know what to do considering it was a six-year-old boy and he was presumably going to push the button himself or someone was going to detonate it for him remotely," Lt Col Accetta said.

The rules of military engagement are easily muddied when a child poses a direct threat, he explained. "What we do if we identify the fact that an adult is wearing a suicide vest is we use whatever force we deem necessary to protect the lives of our soldiers and any civilians. Of course it makes it more difficult - it's a six year-old child."

The date of the incident, the boy's name and information on what happened to him afterwards were not immediately available, Lt Col Accetta said. The Guardian has been unable to independently corroborate the claim.

ISAF has accused the Taliban of intentionally living and fighting in residential areas, capitalising on the international forces' reticence to put ordinary Afghans at risk.

"They will normally intermix with the civilian population with the thought that we won't engage them there, and it's true, we won't do that," Lt Col Accetta maintained. "They are deliberately putting civilians - women and children - at risk by bringing the combat into close proximity with them."

Coalition forces have struggled in recent days to pacify a swell of anger following repeated incidents where innocent civilians have apparently been killed in military operations.

Responding to reports that women and children had been killed in the latest airstrike, a spokesman, Lt Col Charlie Mayo, said: "If civilians had been identified in the area the air strike would not have gone ahead."

He added: "ISAF has demonstrated this restraint on a number of occasions and goes to great lengths to minimise civilian casualties."

Backstory

That the civilian toll in Afghanistan is on the rise is not in dispute. At least 230 people have been killed already this year, including the 25 who died yesterday and seven children killed in an airstrike on Sunday. What is more contentious is who is to blame. Nato accused the Taliban of hiding behind civilians during attacks. But protests against civilian deaths are growing and unsettling the government. President Hamid Karzai has insisted that Afghan authorities be consulted on any airstrikes. He said yesterday's deaths were "difficult for us to accept".