The defence secretary, Philip Hammond, has been asked to launch an urgent inquiry into claims that British forces led a counter-insurgency operation in Afghanistan during which a 12-year-old boy and three teenagers were shot dead while they were drinking tea.

Lawyers acting for the brother of two of the victims have written to Hammond describing an incident on 18 October in the village of Loi Bagh in Nad Ali, Helmand province, where British forces have been based since 2006.

According to statements given to the lawyers by other family members and witnesses, the operation involved Afghan and UK forces, but it was British soldiers – possibly special forces – who were said to have been in the lead.

"We submit that all of the victims were under the control and authority of the UK at the times of the deaths and ill-treatment," states the letter to Hammond.

"The four boys killed all appear to have been deliberately targeted at close range by British forces. All were killed in a residential area over which UK forces clearly had the requisite degree of control and authority."

The four victims are named as Fazel Mohammed, 18, Naik Mohammed, 16, Mohammed Tayeb, 14 and Ahmed Shah, 12.

Britain contributes soldiers to Nato's International Security and Assistance Force (Isaf), which has already confirmed that an operation took place in the village on that date.

The incident has been reported in the Afghan media. Major Adam Wojack, a spokesman for Nato-led forces in Afghanistan, confirmed the "joint Afghan-coalition forces" operation in Nad Ali on 18 October. He said the result was the "killing of four Taliban enemies in action". That claim is rejected by relatives of the victims.

Military sources also said it was unusual for UK forces to take the lead in operations of this kind because the Afghans are supposed to be in control as part of the transition process. The MoD said it would give the claims "full consideration before responding".

According to a statement sent to Hammond on Tuesday by Tessa Gregory, lawyer for Noor Mohammad Noorzai, brother of two of the dead youths, the boys were "shot and killed at close range" in a family guesthouse. Gregory, of the law firm Public Interest Lawyers, obtained written sworn statements from witnesses in a visit to Afghanistan last month. They allege that British soldiers, who were engaged in a joint operation with Afghan forces, hooded some of those arrested despite a ban on the practice.

"The soldiers walked through the village calling at various houses asking to be told where the claimant's brother Fazel Mohammed lived", says Gregory's statement. "It is alleged that the soldiers entered the house of a neighbour dragged him from his bed, hooded him and his son and beat them until under questioning they showed the soldiers the house of Fazel which was across the street."

According to the document sent to Hammond, the families and neighbours "reject outright any suggestion that any of the four teenagers killed were in any way connected to the insurgency. All four were innocent teenagers who posed no threat whatsoever to Afghan or British forces".

Gregory told the Guardian: "On 18 October 2012, during a joint British-Afghan security operation, four innocent Afghan teenagers were shot whilst drinking tea in their family's mud home in Helmand province. Our client, the elder brother of two of the teenage victims, wants to know why this happened. As far as we are aware no investigation into these tragic deaths has taken place. We hope that in light of our urgent representations the Ministry of Defence will act swiftly to ensure that an effective and independent investigation is carried out without any further delay."

In her statement to Hammond, Gregory says: "After the soldiers left, the claimant's family and some neighbours entered the "guesthouse" where they found the bodies of the four teenagers lying in a line with their heads towards the doorway".

The statement adds: "It was clear that the bodies had been dragged into that position and all had been shot in the head and neck region as they sat on the floor of the guesthouse leaning against the wall drinking tea.."

Gregory says the British soldiers involved in the operation are bound by the European Convention of Human Rights which enshrines the right to life and outlaws inhumane treatment. Unless the MoD could show it has carried out a full investigation, lawyers representing the victims' families will ask the high court to order one.

An MoD spokesman said: "The Ministry of Defence received details of these allegations on Tuesday in a letter from a UK firm of solicitors on behalf of an Afghan national and will give them full consideration before responding. The 'letter before action' is the first stage of seeking a judicial review and requires the MoD to reply within 14 days, providing a reasonable opportunity to consider the claim and whether there is a case to answer."

The MoD said protection of the Afghan civilian population is at the core of Isaf's military strategy in Afghanistan and that, unlike the insurgency, Isaf and UK forces place a high priority on protecting civilians during combat.

• This article was amended on 13 December 2012 to correct the last paragraph, which had been mangled in the production process.