KABUL -- Six British soldiers were killed in Helmand province when an explosion hit their armored vehicle, Britain's Ministry of Defense said yesterday. If was the biggest loss of life for British forces in the country since a plane crash in 2006.

British Prime Minister David Cameron said the deaths marked a "desperately sad day for our country."

The attack Tuesday is certain to fuel calls for the acceleration of a planned withdrawal of all U.S.-led coalition troops from Afghanistan by the end of 2014.

Washington has grown frustrated with the administration of President Hamid Karzai, who has been making increasing demands of America in order to sign a deal that will allow some troops to remain past 2014, mainly in a counterterrorism and training role.

Helmand, in the south, has been the deadliest province by far for coalition troops since the Afghan war started over a decade ago. Most of Britain's 9,500 soldiers are based there, and the province also has thousands of U.S. troops.

The Taliban have fought fiercely for control of Helmand because it accounts for about half of all poppy production in Afghanistan.

Poppy, the main ingredient in making opium, has been a significant source of revenue for the militants.

So far this year, 54 NATO troops have been killed in Afghanistan, including 38 from the United States and four from Britain.

Britain has lost more troops in Afghanistan, 404 after Tuesday's killings, than any other country except the United States, which has counted at least 1,780 deaths as a result of the U.S.-led invasion in late 2001, according to an Associated Press count.

In 2006, a Nimrod aircraft crashed in Afghanistan, killing 14 British service members.

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