Six British soldiers were killed when their armored vehicle was hit by an explosion while they were on patrol in Helmand province in southern Afghanistan, a British military official said on Wednesday, March 7, Reuters reported.

The incident marked the biggest single loss of life for British troops in Afghanistan since 2006 and takes the overall death tally for British forces there to 404 since the 2001 U.S.-led invasion to oust the Taliban.

Mohammad Ismail Hotak, a senior Afghan police official in Helmand, said the explosion was caused by a roadside bomb, the deadliest weapon used by insurgents.

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

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.