The Defence Secretary Bob Ainsworth admitted today there was "gloom and worry" about the British death toll in Afghanistan as the seventh UK fatality in a week was announced.
But he insisted troops on the ground have a sense of momentum and rejected comparisons with the Vietnam conflict.
Mr Ainsworth said: "There is, of course, gloom and worry back here in London with the numbers of people that we've lost. If people weren't (worrying), there would be something seriously wrong with them.
"But when you go out to Afghanistan, as I did last weekend, there is a very real sense of momentum."
His comments come after a difficult few days for British forces in Afghanistan, with seven deaths in the first week of July alone.
They included Lieutenant Colonel Rupert Thorneloe, the commanding officer of the 1st Battalion Welsh Guards, who became the most senior Army officer to be killed on operations since the Falklands War.
This morning the Ministry of Defence announced the latest British fatality, a soldier from the Light Dragoons who was killed in Helmand Province yesterday.
At the same time, senior figures have cast doubt on the effectiveness of the international campaign against the Taliban.
But Mr Ainsworth insisted the troops were clear about their mission and were making progress.
"There is no doubt in their minds a) that they are achieving something, and b) that they are there for a purpose and that purpose is - boil it down - to help the Afghans and to protect national security," he told BBC Radio 4's Today programme.
"There is no misunderstanding on their part that they are making progress, that they are there for a good cause. No doubt whatsoever."
He said the Taliban's abilities had been significantly "degraded" while the Afghan government was better able to "reach its own people".
When it was put to him that the conflict was becoming "another Vietnam", Mr Ainsworth said: "I don't accept that. We have made considerable progress."
The defence secretary will today make his first major speech since his appointment in last month's Cabinet reshuffle.
He is expected to set out the rationale for Britain's continuing engagement in the region in his address to the Chatham House foreign affairs think tank in London.
The British force is currently engaged in an intense operation, codenamed Operation Panther's Claw, targeting the Taliban in the central Helmand river valley.
At the same time a parallel operation by 4,000 US marines, who have now been sent to reinforce the British mission in Helmand, is seeking to clear the southern part of the valley.Reuse content