LONDON (AP) — Britain’s defense secretary insisted on Thursday that the country’s troops are committed to their mission in Afghanistan, after the number of U.K. personnel killed in the conflict passed 400.

Ministers have expressed fears that the deaths of six soldiers killed when an explosion hit their armored vehicle in the southwestern Helmand province on Tuesday evening would prompt new concern over the 11-year-long campaign.

The incident was the biggest loss of life for British forces in Afghanistan since a plane crash in 2006. It took Britain’s death toll in Afghanistan since 2001 to 404.

"The people on the ground are acutely conscious of the risks that they are running, but they are also incredibly proud of the job that they are doing and rightly so, and hugely satisfied by the level of public support that they have back home," Defense Secretary Philip Hammond told ITV television.

He said morale remains "extremely high, and it’s high because the servicemen and women there know that they are doing a job and are doing it well and that is their professional commitment to get that job done."

Britain’s defense ministry named those killed as Cpl. Jake Hartley, 20; Pvt. Anthony Frampton, 20; Pvt. Christopher Kershaw, 19; Pvt. Daniel Wade, 20; Pvt. Daniel Wilford, 21; and Sgt. Nigel Coupe, 33.

Hammond said that full details of the incident were not yet established, but that it appeared "a massive" roadside bomb had exploded and "had a catastrophic impact on the vehicle."

Gen. David Richards, head of Britain’s armed forces, acknowledged that the military is likely to suffer further losses before the end of the international mission in Afghanistan at the end of 2014.

"As progress continues, the work of our servicemen and women will draw down, but our efforts will endure," he wrote in an op-ed article in the Times of London. "Sadly, as we hold that course, it is likely that others will lose loved ones."

The attack is expected to lead to new calls to accelerate the planned withdrawal of all U.S.-led coalition troops from Afghanistan by the end of 2014.

British Prime Minister David Cameron said Wednesday he planned to discuss the drawdown in talks next week in Washington with President Barack Obama.