A "rogue" Afghan policeman gunned down five British soldiers at a checkpoint in Afghanistan, officials said on Wednesday, fuelling growing questions about the Afghan mission.

The killings underscored the danger and complexity of the war in Afghanistan and heightened concerns as the government is hoping to shift its troops away from front-line fighting to training local forces to handle security themselves.

The soldiers died of gunshot wounds in the attack in the Nad-e'Ali district of Helmand Province on Tuesday, the Ministry of Defence in London said in a statement.

The soldiers had been mentoring Afghan police and living inside the checkpoint.

"Initial reports suggest that it was a member of the Afghan National Police who fired without warning. The incident happened at a checkpoint and he fired before anyone could respond," said a military spokesman.

"This matter is subject to a parallel investigation between the British Military Police and the Afghan Police."

The spokesman said: "It's our understanding that one individual Afghan National Policeman, possibly in conjunction with another, went rogue. His motives and whereabouts are unknown at this time.

"Every effort is now being put into hunting down those responsible for this attack."

Three of the soldiers were from the Grenadier Guards and two were from the Royal Military Police. Their families have been informed.

The incident brings to 229 the number of troops who have been killed since operations in Afghanistan began in October 2001. Of these, at least 193 were killed as a result of hostile action.

With 94 soldiers now killed on active service this year -- 93 in Afghanistan and one in Iraq -- it makes 2009 the bloodiest year for the armed forces since 1982 and the Falklands War with Argentina.

Prime Minister Gordon Brown has faced persistent questions this year over the scope and purpose of the military's role in Afghanistan as the death toll surged, and whether troops were well-enough equipped.

"The death of five brave soldiers in a single incident is a terrible loss," Brown said.

"My thoughts, condolences and sympathies go to their families, loved ones and colleagues. I know that the whole country, too, will mourn their loss."

Defence Secretary Bob Ainsworth said he was "extremely saddened" by the deaths.

"It continues to be a difficult year in Afghanistan for our brave people who are operating within the most challenging area of the country," he said.

"We owe it to them to show the resolve that they exhibit every day in building security and stability in Afghanistan and protecting the UK from the threat of terrorism."

Britain has around 9,000 troops based in Afghanistan's troubled southern Helmand province, where they are battling Taliban insurgents. Brown has conditionally pledged another 500 troops for the mission.

Kim Howells, the former Foreign Office minister with responsibility covering Afghanistan, told the BBC that the killings undermined strategy of building up the local forces.

"There are many people who have argued that there is only one way out of this for Britain and America and that is to train up the Afghan army and police force so that they can become responsible for their own security," he said.

"This is a real blow because it strikes right at the heart of that policy."

Howells earlier broke with government policy with a newspaper article calling for the phased withdrawal of troops.