Defence Secretary Des Browne says a "long term commitment" in Afghanistan is essential to prevent its return to being a training ground for terrorists.
He was speaking following the death of another British soldier in the volatile Helmand province over the weekend.
Mr Browne said UK forces were doing an "exceptionally good job" in Afghanistan, but they needed to be supported by the growth of governance in the country.
He told BBC Radio 4's Today programme: "I have never underestimated the degree of difficulty we face here, but we are making progress."
Mr Browne said the troops were fighting to ensure that Afghanistan has the "best future" it can have.
He said: "We are in Afghanistan... to ensure that a country which has gone through three decades or more of dreadful violence, lost two million of its own people securing its own freedom, has the opportunity that its citizens deserve to have the best future they can have in an already challenging environment.
"And to ensure that this ungoverned space - as it had become after 30 years of that sort of violence, and had become a training ground for terrorists - never again becomes a training ground for terrorists."
Mr Browne conceded that production of opium poppy had increased, but said successes were being made in other aspects of Afghan life.
"We are beginning to make a difference but it's not an easy thing to do," he said.
"The fact of the matter is it is a long term commitment and our people are doing an exceptionally good job there but it has to be complemented by the growth of governance from the Afghans themselves. They have to take increasing responsibility not just for security but to hold the stability which we can generate."
