Six British soldiers went missing in Afghanistan after their armored vehicle hit a landmine in the dangerous southern Helmand province. They're believed to be dead.

I have the tragic duty to report that six soldiers are missing, believed killed, during a security patrol, stated Lt. Col. Gordon MacKenzie, a spokesman for a task force of soldiers in the province, on Wednesday.

The six soldiers, five from the 3rd Battalion [of] the Yorkshire Regiment and one from the 1st Battalion [of] The Duke of Lancaster's, were on patrol in a Warrior armored fighting vehicle when it was caught in an explosion in the Task Force Helmand area of operations, said MacKenzie.

Not many details about the blast are known, but if the soldiers were killed the incident marks the worst single loss of British troops in the country since 2006.

The British Warrior vehicle was traveling over unpaved roads in the Nar-e-Saraj area of the province, where British troops have been training local police officers and security forces for about a year, according to CNN.

The blast could have been caused by a legacy device, a landmine left over from the war with the Soviets in the 1980s, which could have been replanted by insurgents, the news outlet reported.

Including the six, a total of 404 Britons have died in Afghanistan since the invasion in 2001.

It is a reminder of the huge price that we are paying for the work we are doing in Afghanistan and the sacrifice that our troops have made and continue to make, British Prime Minister David Cameron told the BBC.

I do believe it's important work for our national security right here at home but of course this work will increasingly be carried out by Afghan soldiers and we all want to see that transition take place.

The UK has 9,500 soldiers in Afghanistan, a number that will decrease incrementally as the 2014 troop withdrawal date nears.

It's disappointing to have this at the end of the tour but at this stage in the tour with how much progress we've made, we're able to contextualize this, a senior British military source in Helmand told Reuters.

A total of about 2,800 military personnel from all nations have died in Afghanistan since the war started in October 2001. This year, 54 NATO troops have been killed, including four others from Britain and 38 Americans.