Six British soldiers are believed dead after their armored vehicle was struck by a roadside bomb.

The soldiers were patrolling in Helmand province in southern Afghanistan Tuesday evening when the blast struck. If the deaths are confirmed, it would be the single deadliest incident for British troops since a helicopter crash in 2006 killed 14.

British Prime Minister David Cameron mourned the loss, but said the mission in Afghanistan remained important for British national security.

“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.''

Defense Secretary Philip Hammond also condemned the attack, but said it would “ultimately fail to derail” the mission.

Meanwhile negotiations continue regarding the continued U.S. presence in Afghanistan after international combat troops leave the country in 2014.

Afghan President Hamid Karzai said Tuesday there has been some progress in outlining the U.S. role and said he hoped both countries could reach a “positive result” for the benefit of Afghanistan.

Afghan officials had earlier said that discussions on a long-term U.S.-Afghan strategic agreement had stalled due to disputes over the transfer of American-run detention centers and night raids conducted by U.S. and NATO forces.