Six British soldiers were killed when a massive explosion hit their armoured vehicle in Afghanistan, the military said Wednesday, taking the British toll in the war against Taliban insurgents to more than 400.

The soldiers were on patrol in the restive southern province of Helmand, where most British troops are based, when the blast hit their heavily-armoured and tank-tracked Warrior fighting vehicle Tuesday, the British defence ministry said.

Early reports listed the soldiers as "missing, believed killed", but military sources said later the men were dead and that it had taken time to recover the vehicle.

"We were on a joint patrol mission in Nahre Saraj district near Lashkar Gah city last night when a British armoured vehicle ahead of us hit a landmine, killing six soldiers," an Afghan army corps commander in Helmand, Sayed Malook, told AFP.

A witness said the vehicle burned all night.

"The tank of the foreign forces has been totally burned down -- it was in flames all last night," a resident of the district, Abdul Ali, told AFP.

"It wasn't turned over, but it is charred now."

A military source in Afghanistan said the possibility that the vehicle had hit a Soviet-era mine had not been ruled out.

Old mines remain a threat in a country that has suffered 30 years of war, including a 10-year occupation by Soviet forces in the 1980s.

Taliban insurgents said in a statement on their website that the vehicle -- described as an American "tank" -- had been blown apart by an improvised explosive device, or homemade bomb, but stopped short of claiming responsibility.

British Prime Minister David Cameron described the explosion as marking a "desperately sad day for our country".

"Every death and every injury reminds us of the human cost paid by our armed forces to keep our country safe," Cameron told parliament.

"Our mission in Afghanistan does remain vital to our national security," he continued.

"We're there to prevent that country from being a safe haven to Al-Qaeda, from where they might plan attacks on the UK or our allies."

Before the explosion, 398 British forces personnel had died in Afghanistan since the start of operations in October 2001, when a US-led invasion overthrew the Taliban regime over its harbouring of Al-Qaeda leader Osama bin Laden.

It is the biggest British loss of life in a single incident in Afghanistan since a Nimrod aircraft crashed in 2006 after leaking fuel made contact with a hot air pipe, killing 14 crew.

Britain has around 9,500 troops with the NATO force of some 130,000 in Afghanistan, but Cameron announced in July that this would be reduced by 500 to around 9,000 this year.

Along with the rest of the NATO coalition, Britain is due to end combat operations in Afghanistan by late 2014, transferring responsibility for security to Afghan forces.

Cameron stressed the need for a political settlement with the Taliban.

"(Britain must) send a very clear message to the Taliban that, whether it is our troops who are there or whether it is Afghan troops who are there, they will not win on the battlefield -- they never win on the battlefield," he said.

"Now it is time for a political settlement to give this country a chance of peaceful progress."

Britain has lost more lives than any country with troops involved in the conflict except the United States.

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