Six British soldiers were killed when a massive explosion hit their armored vehicle in southern Afghanistan.
The soldiers were on patrol near the capital of Helmand province, Lashkar Gah, Tuesday evening when the blast happened. It was the single deadliest incident for British troops in Afghanistan since a helicopter crash in 2006 killed 14.
NATO said the troops were believed killed following an improvised explosive device attack. Afghan officials said the vehicle hit a landmine.
The deaths bring the number of British troops killed in Afghanistan since 2001 to 404. 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," said Cameron. "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.
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 believed killed Tuesday 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.
Elsewhere in Afghanistan Wednesday, officials say four civilians were killed when a bomb planted on a motorcycle exploded in the southern province of Kandahar.
At least nine other civilians were wounded in the attack, which took place in the border town of Spin Boldak.
Some information for this report was provided by AP, AFP and Reuters.