In the worst "friendly fire" incident involving British forces in the country, an American F-15 long-range strike aircraft dropped a single 500lb bomb killing the soldiers from 1st Battalion the Royal Anglian Regiment.

The investigation will need to determine whether the accident was the result of a communications or technical failure, why an American rather than a British plane was involved, and why such a relatively big bomb was dropped close to British positions on the ground.

The soldiers were part of a 60-man patrol deployed to disrupt Taliban movements north-west of Kajaki, the site of a hydroelectric dam under repair and potentially significant irrigation projects for Helmand province. At about 6.30pm local time on Thursday the patrol was attacked by Taliban fighters and came under heavy gunfire from several directions.

In a statement yesterday the Ministry of Defence said: "During the intense engagement that ensued, close air support was called in from two US F-15 aircraft to repel the enemy. A single bomb was dropped and it is believed the explosion killed all three soldiers who were declared dead at the scene."

The next of kin had been informed, the MoD said. The two injured soldiers, including one critically ill, were evacuated by helicopter to the medical facility at Camp Bastion. Nine soldiers from the battalion have been killed in southern Afghanistan over the past four months.

The latest friendly fire deaths are thought to be the second incident involving British soldiers being killed by Americans in Afghanistan. Eight British military personnel have been killed by US fire in Iraq since the start of the war in 2003.

Yesterday's incident brings the total number of British troops killed while on operations in Afghanistan since 2001 to 73. Fifty have been killed in action.

Lt Colonel Charlie Mayo, the British army spokesman in Helmand, said: "There are a handful of different reasons why this tragic incident has happened and we are not in a position at the moment and I don't think we will be for some time to find out exactly what has happened."

The Ministry of Defence was criticised by MPs earlier this year for delays in installing new technology in systems designed to prevent friendly fire. However, in past friendly fire incidents human error has often been the cause.

The MoD said British troops in Afghanistan had frequently relied on US and British air support to get them out of trouble. The government made clear it is anxious that the incident did not further exacerbate relations and is stressing there is no assumption that the US is to blame. It is emphasising that there are parallel investigations: while the US will investigate the role of its pilots, the British will check whether the British soldiers may have given the wrong coordinates.

Des Browne, the defence secretary, said the investigation into the incident would be "thorough", adding that he would not "indulge in any speculation" about the causes. He described such incidents as "rare". He added: "We go to extraordinary lengths to ensure these things don't happen but at the end of the day combat environments are very complex environments. Human error is always a possibility."

The shadow defence secretary, Liam Fox, said: "It is wrong to jump to any conclusions. Many issues including combat identification systems will need to be considered in the investigation."

In Washington, Lt Colonel Todd Vician, a Pentagon spokesman, said: "We express deep condolences to the families and loved ones of the British soldiers who died and we wish those who were injured a full recovery." He added: "Close air support in a combat environment is tremendously challenging and our forces train and prepare for operations to eliminate incidents to the greatest extent possible."

The Pentagon is sensitive about friendly fire incidents, particularly those involving British troops. It regards US airmen as unfairly stereotyped as trigger-happy.

It is still angry over the leaking earlier this year of a classified video it sent to Britain for the inquest into the death of Lance Corporal Matty Hull, killed by a US air attack in Iraq in 2003.

Relations between the US and Britain have been strained in recent weeks, with the British criticising US bombing in Afghanistan for alienating the local population after of a rising toll of civilian death. US officers have been critical of the British performance in southern Iraq.

The Nato-led International Security Assistance Force, Isaf, said it had procedures in place to minimise the risk of friendly fire incidents. Spokeswoman Lt Col Claudia Foss said: "Isaf is committed to finding out exactly how this tragedy occurred and how similar incidents can be avoided."