(ROUGH CUT ONLY - NO REPORTER NARRATION)

British Prime Minister David Cameron said on Wednesday it was a "desperately sad" day for the British armed forces as six British soldiers were missing and believed to have been killed after their armoured vehicle was hit by an explosion while they were on patrol in the Helmand area of southern Afghanistan.

"This is a desperately sad day for our country, and desperately sad of course for the families concerned. It is a reminder of the huge price that we're paying for the work we're doing in Afghanistan, the sacrifice that our troops have made and continued to make. I do believe that 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," he said.

A defence ministry statement said the soldiers, five from the 3rd Battalion the Yorkshire Regiment and one from the 1st Battalion The Duke of Lancaster's Regiment, were on a mounted patrol when their Warrior Armoured Fighting Vehicle was struck.

If the soldiers are dead, the incident will be the biggest single loss of life for British troops in Afghanistan since 2006 and take the overall death tally for British forces there to 404 since the 2001 U.S.-led invasion to oust the Taliban.

Britain has some 9,500 soldiers in Afghanistan, a number due to be reduced in phases as it ends combat operations in the next two years.