The Royal Air Force Nimrod MR2 reconnaissance plane, on a NATO mission, came down in Kandahar province in southern Afghanistan on Saturday due to a technical problem, a NATO spokesman in Kabul said, stressing that it was not shot down.

The crash caused the biggest single loss of British troops in Afghanistan or Iraq since the US-led war on terror was launched in November 2001. Twelve RAF personnel, a Royal Marine and a British Army soldier were killed.

The incident brings the number of British armed forces personnel deaths in Afghanistan since the start of operations against the hardline Taliban regime in 2001 to 36, including 15 in combat.

Six soldiers were killed last month alone and the steadily rising death toll has left some wondering how much longer Britain can tolerate such losses.

Rejecting suggestions that it was "mission impossible", Defence Secretary Des Browne said in a television interview that people needed to understand why British troops were in the country.

"We're there in the context of a United Nations resolution," he said.

"I know that the south (of Afghanistan) is a very difficult and dangerous place and our own experience there has suggested that."

However, "the developed world cannot afford for Afghanistan to become a training ground for terrorists again."

Browne added: "We have secured the site and the investigation has begun.

"The indications, both before the crash and since, suggest that this has been a dreadful, tragic accident."

Prime Minister Tony Blair said the crash was a "tragedy" which would "distress the whole country".

"British forces are engaged in a vital mission in Afghanistan and this terrible event starkly reminds us of the risk that they face daily," he said.

Britain has nearly 4,000 troops in Helmand as part of a NATO-led force working to bring security to the restive southern province in order to allow reconstruction work to take place.

But newspapers revealed that rather than being on mere peacekeeping and security duties, Britain was officially at war in Afghanistan.

The Sunday Telegraph and the Sunday Mirror said that under new rules of engagement, commanders now have "war fighting" rules, giving them the legal authority to launch air strikes against suspected Taliban strongholds, conduct ambushes and order pre-emptive attacks against insurgents' camps.

A Ministry of Defence spokesman told the Sunday Mirror that the situation was being "kept under constant review".

With 7,200 British troops also in Iraq, accusations -- denied by the government and military top brass -- have mounted in recent months that troops are overstretched and ill-equipped.

Brigadier Ed Butler, the commander of British forces in Afghanistan, said troops were "delivering over and above" and the recent spike in deaths had caused "profound personal devastation" among families, friends and colleagues.

Defence expert Major Charles Heyman said the crash would inevitably cause military and political problems for Blair's government.

"It's always going to have an effect on domestic public opinion: people are going to say, 'What are our troops doing there?'

"Overall, it begins to look like an unlucky operation."

He said Pakistani contacts told him "the hills are crawling with well-armed and well-trained Taliban".

"Given this fact, the government has two options: massive reinforcement or order them to withdraw."

The Ministry of Defence later on Sunday released the names of the dead.

The 12 RAf personnel were: Flight Lieutenants Steven Johnson, Leigh Anthony Mitchelmore, Gareth Rodney Nicholas, Allan James Squires and Steven Swarbrick, Flight Seargents Gary Wayne Andrews, Stephen Beattie, Gerard Martin Bell and Adrian Davis, and Seargents Benjamin James Knight, John Joseph Langton and Gary Paul Quilliam.

Marine Joseph David Windall and Lieutenant Corporal Oliver Simon Dicketts from the Parachute Regiment were also killed.

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