Three British soldiers have been killed in Afghanistan by American fighter jets in one of the worst so-called friendly fire incidents in recent years.

The three men from the 1st Battalion the Royal Anglian Regiment were killed when they came under fire from one of two American F15 aircraft called in to support British troops who were fighting near the Jajaki dam in Helmand province.

The Ministry of Defence said three soldiers were declared dead at the scene following the incident which happened at about 6.30pm local time on Thursday.

Two others were taken to Camp Bastion with serious injuries.

Lieutenant Colonel Charlie Mayo, spokesman for British troops in Helmand, said an investigation had been launched.

He told Radio 4's The World at One: "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 incident led to calls for Prime Minister Gordon Brown to reappraise the government's strategy in Iraq and Afghanistan.

Menzies Campbell, leader of the Liberal Democrats, said: "This is a tragic accident and will require full investigation."

Mr Campbell wrote to Mr Brown earlier this week saying the level of British casualties in Iraq and Afghanistan is "unacceptable".

He added: "There are persistent reports that there will be a reduction in the number of British forces deployed to Iraq. It is time to set a framework for the complete withdrawal of all of our forces.

"The need to do so is underlined by the situation in Afghanistan where our forces are at full stretch. Their position would be made much easier if we were not engaged simultaneously in two such demanding deployments."

Defence expert Major Charles Heyman, editor of the Armed Forces of UK annual book, last night said it was too early to be critical of the US involvement in the incident. People must wait "until all the facts of an investigation are known".

He added, however: "This sort of offensive air support is a substitute for troops on the ground. We have 7000 British troops in Helmand province, which is about three times the size of Wales. To do the job properly we need three times that number.

"The troops are doing a wonderful job for the number they have on the ground. But it is too much to expect them to be able to control this huge area, which has a massive border with Pakistan, where the Taliban can come and go as they please."

The incident is believed to be the second of its kind involving British troops in Afghanistan.

The MoD is still investigating reports that Royal Marine Jonathan Wigley, 21, was the victim of allied fire when he was killed in Helmand in December.

A string of friendly fire deaths in Iraq prompted defence officials to propose special combat simulators to train troops involved in joint operations.

Eight UK personnel have become accidental victims of allied forces since the start of the conflict in 2003, according to the MoD. The most high-profile case involved Lance Corporal Matty Hull, killed after a US "tankbuster" aircraft fired on two armoured vehicles in March 2003.

L-Cpl Hull, 25, of the Household Cavalry Regiment, died from multiple injuries in his blazing Scimitar tank.

Other UK soldiers who survived the attack criticised the US pilot for apparently failing to recognise coalition identification aids.

The US initially refused to release a classified cockpit recording of the incident, only relenting when a British newspaper obtained a copy and published it on the internet.

The Americans have also repeatedly refused to send personnel involved in friendly fire deaths to UK inquests.

L-Cpl Hull's death had echoes of an incident during the 1991 Gulf war when nine British soldiers died after their vehicles were attacked by US tankbusters.

The US embassy in London said in a statement yesterday: "The United States expresses its deep condolences to the families and loved ones of the soldiers who died, and we wish those who were injured a speedy recovery."

Defence Secretary Des Browne said: "I do not want us to get into a situation where we're blaming each other when, as a matter of fact, US air support has saved our people's lives on many occasions, particularly over the last four months in that very theatre."

The latest deaths bring to 73 the number of British troops killed in Afghanistan since the US-led invasion in November 2001.