KABUL - A "rogue" Afghan policeman shot dead five British soldiers in Afghanistan Wednesday, raising new questions about the safety of coalition troops as world leaders work to boost training of local forces.
The Afghan attacker on Tuesday opened 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 -- before fleeing the scene.
The soldiers killed had been mentoring Afghan police and living at the checkpoint. Brown condemned the incident as "terrible and tragic".
"What we know is that the Taliban have claimed responsibility for this," Brown told lawmakers in London.
"It may be that the Taliban have used an Afghan police member or that they have infiltrated the Afghan police force," he added.
The killings underscored the increasing complexity of the eight-year 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.
NATO Secretary General Anders Fogh Rasmussen refused to budge on his stance that NATO training of Afghan security forces must lie at the heart of any future exit strategy.
"I consider this an isolated incident and it does not change my position," Rasmussen said in an interview with AFP.
A British military spokesman said initial reports suggested that the Afghan policeman fired without warning.
"This matter is subject to a parallel investigation between the British military police and the Afghan police," he 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."
British opposition leader David Cameron said the attack raised serious questions about the safety of British troops mentoring Afghan security forces.
But Brown said training programmes were crucial because they showed that international forces were helping Afghanistan to manage its own affairs.
"It is an essential element of the whole coalition strategy that we train up the Afghan forces so that they themselves are able to take over the security of this country, and that we will continue to do," he said.
General Stanley McChrystal, commander of the more than 100,000 NATO and US troops in Afghanistan, said the incident was being investigated jointly with Afghan authorities.
"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 said in a statement."
Afghan 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.
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.
Brown has faced persistent questions over the scope and purpose of the British role in Afghanistan and whether troops were well-enough equipped.
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 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."
Meanwhile Karzai was criticised by the US military for failing to deal with corruption among Afghan leaders.