LASHKAR GAH (Reuters/A FP) - Six British soldiers were killed when their armoured vehicle was hit by an explosion in Helmand province in southern Afghanistan, British military officials said on Wednesday, taking the British toll to 404 since the 2001 US-led invasion to oust the Taliban.

The soldiers, five from the 3rd Battalion the Yorkshire Regiment and one from the 1st Battalion The Duke of Lancaster’s Regiment, were on a mounted patrol on Tuesday when their Warrior Armoured Fighting Vehicle was struck, the British Ministry of Defence said.

“One of the Warriors suffered catastrophic damage in the course of that explosion,” Brigadier Patrick Sanders, commander of Task Force Helmand, told journalists in the provincial capital Lashkar Gah. The attack, on Helmand’s border with the unruly Kandahar province, marked the biggest single loss of life for British troops in Afghanistan since 2006. Britain is the second largest contributor to the Nato-led war after the US. The Britons’ vehicle was hit by a roadside bomb, the deadliest weapon used by insurgents, said senior Afghan police official Mohammad Ismail Hotak in Helmand.

“It is a reminder of the huge price that we are paying for the work we are doing in Afghanistan and the sacrifice that our troops have made and continue to make,” British Prime Minister David Cameron told the BBC.

“I do believe 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.” Britain has around 9,500 soldiers in Afghanistan, a number due to be reduced in phases as it ends combat operations in the next two years.

Task Force commander Sanders said Tuesday’s attack had “a significant impact emotionally. These are comrades.”

But describing the assault as a “one-off,” he stressed that such a large loss of life runs contrary to the trend of the last eight or twelve months in Helmand.

Referring to the fact the number of British deaths in the Afghan war had exceeded 400, a senior British military source in Helmand said: “I don’t think soldiers see these round numbers, psychological milestones.”

“It’s disappointing to have this at the end of the tour but at this stage in the tour with how much progress we’ve made, we’re able to contextualize this,” he added.

Separately, four civilians were killed and 10 injured Wednesday when a remotely controlled motorbike bomb exploded in an Afghan town on the border with Pakistan, police said.

Officials blamed the attack in the town of Spin Boldak in Kandahar province on the hardline Taliban. “There was a motorbike bomb blast against border police in Spin Boldak. Four civilians, one of them a woman, were killed and 10 others were injured,” said Janan, a rapid-reaction police unit commander who uses only one name.

The interior ministry confirmed the deaths and blamed the blast on the “enemies of Afghanistan” - a reference to the Taliban.