British Ambassador to Afghanistan William Patey said six British soldiers stationed in Helmand province were missing and presumed dead.
"In our endeavor to build a stable and secure Afghanistan, we will continue to work to ensure that their sacrifice will not be in vain," said Patey.
British army Lt. Col. Gordon Mackenzie, a spokesman for Task Force Helmand, said the six soldiers were on patrol in an armored fighting vehicle when they were "caught in an explosion."
British forces are working on provincial reconstruction and training in southern Afghanistan. If confirmed, their deaths would push the number of British soldiers killed in Afghanistan to more than 400 since the war began in 2001.
The British government didn't explain why they were unable to confirm the status of the six soldiers in Helmand.
International forces said they'd start handing over security responsibility to their Afghan counterparts next year in an effort to wrap up the mission by 2014.
Anti-Western demonstrations erupted in Afghanistan erupted last year following reports of the mishandling of the Koran at a military base.
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