A "rogue" Afghan policeman shot dead five British soldiers in Afghanistan, precipitating an immediate inquiry and fuelling further questions about the war back in London, officials said on Wednesday.
The attacker fled after opening fire at a checkpoint in the Nad Ali district of southern Helmand province, where the vast majority of Britain's nearly 9,000 troops are based, on Tuesday and is being hunted down, the British defence ministry said.
The soldiers who were killed had been mentoring Afghan police and living at the checkpoint.
The killings underscored the increasing danger and complexity of the war in Afghanistan, where Western military and political leaders have put NATO training of Afghan security forces as the heart of any future exit strategy.
"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 British military spokesman.
"This matter is subject to a parallel investigation between the British military police and the Afghan police.
"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."
General Stanley McChrystal, commander of the 100,000 NATO and US troops in Afghanistan to train local security forces and fight Taliban insurgents, said the incident was being investigated jointly with Afghan authorities.
"I have spoken to the minister of interior, Minister (Mohammad Hanif) Atmar, who shares my deepest regret for this incident and he gave me his assurance that this incident will be fully and transparently investigated," he said.
"We will not let this event deter our resolve to building a partnership with the Afghan National Security Forces to provide for Afghanistan's future," the general added in a statement.
President Hamid Karzai ordered an investigation and instructed interior ministry officials to "bring those responsible to justice".
Four of the soldiers were killed outright and a fifth died later of wounds, the NATO-run International Security Assistance Force (ISAF) said.
Six other soldiers were wounded, the statement added.
"Every effort is now being put into hunting down those responsible for this attack," said the British military spokesman.
Three of the soldiers were from the Grenadier Guards and two were from the Royal Military Police.
The attack brings to 229 the number of British 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 British soldiers now killed on active service this year -- 93 in Afghanistan and one in Iraq -- 2009 is the bloodiest year for the British armed forces since 1982 and the Falklands War with Argentina.
Prime Minister Gordon Brown has faced persistent questions over the scope and purpose of the British military's role in Afghanistan 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."
Britain has around 9,000 troops based in troubled Helmand province, where they are battling Taliban insurgents. Brown has conditionally pledged another 500 troops for the mission.
Kim Howells, a former British minister with responsibility covering Afghanistan, told the BBC that the killings undermined British strategy to build 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 British troops.