Six British soldiers were killed after their armored vehicle hit a roadside mine in southern Afghanistan.

A NATO statement says the soldiers were on patrol near the capital of Helmand province late Tuesday when the blast occurred. It was the single deadliest incident for British troops in Afghanistan since 2006, when a helicopter crash killed 14 people.

The deaths bring to 404 the number of British troops killed in Afghanistan since the beginning of the war in late 2001. Britain has had the second highest number of casualties in the 10-year war after the United States.

British Prime Minister David Cameron mourned the loss Wednesday, 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.

In neighboring Kandahar province Wednesday, four Afghan civilians were killed and nine others wounded when a bomb planted on a motorcycle exploded in the border town of Spin Boldak.

The commander of the coalition forces in Afghanistan, U.S. General John Allen, joined Afghan President Hamid Karzai in condemning the deaths. Allen said “blatant murdering of Afghan civilians must stop, and the Taliban leadership needs to hold their own members accountable for their actions against the innocent.”

Britain's ambassador to Afghanistan, William Patey, praised the role of British troops in helping build up Afghan security forces and strengthen governance and development in the country. He said the six British soldiers whose vehicle hit the mine were working closely with a British-led provincial reconstruction team in Helmand.

Britain has around 9,500 troops in Afghanistan, most of them in Helmand province, and plans to withdraw several hundred by the end of this year. All international combat troops are set to leave Afghanistan by the end of 2014.