KABUL, Afghanistan (AP) — Six British soldiers were believed killed after an explosion hit their armored vehicle in southwestern Afghanistan, Britain’s Ministry of Defense said Wednesday. If confirmed, it would be the biggest loss of life for British forces in the country since a plane crash in 2006.

The soldiers were on patrol in Helmand province at the time of the blast Tuesday evening. Britain’s defense secretary said efforts were under way to recover the vehicle and identify the soldiers.

The deaths were 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. The war has become increasingly unpopular in the United States and among its NATO partners in Europe.

Washington has also 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 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 is the main ingredient in making opium and 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 — 398 until Wednesday — than any other country except for 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. The AP count of U.S. deaths is six less than the Defense Department’s tally. At least 1,484 U.S. military service members have died in Afghanistan as a result of hostile action, according to the military’s numbers.