Mr Ainsworth: first visit to Afghanistan since his promotion
Supporting UK troops in Afghanistan is the highest priority for the defence budget, Defence Secretary Bob Ainsworth has said.
But he said this needed to be balanced against changing threats the UK faced in an uncertain world.
Mr Ainsworth was speaking following his first trip to Afghanistan after being appointed earlier this month.
His comments come amid talk of defence spending cuts, possibly affecting the Trident nuclear deterrent.
Mr Ainsworth said the economic downturn had put a strain on resources.
"Everybody knows that there is a recession, everybody knows that there's impact on government across the piece," he said.
"We will have to ensure that the funds that we have got are spent on our highest priorities - and we do that all the time."
He said they considered the priorities on an ongoing basis, taking account of current operations.
"Obviously our highest priority has got to be our people who are doing the hard yards and the hard fighting in Afghanistan, and making sure that they are as well catered for as they can be.
"But we have got to balance that against the uncertain world that we face. The threat changes. If one looks back, it's changed hugely over time and no-one can be certain that that won't happen again.
"No-one believed at the end of the 1990s that we would have troops involved the length of time we had in the kind of operation we had in Iraq.
"No-one would ever have believed that we would have 9,000 troops in Afghanistan. It was completely and utterly unpredictable."
He reiterated that Parliament had voted in favour of renewing the Trident submarine-based deterrent, but noted some details of the replacement system had been left undecided. There are some calls, however, for Trident to be scrapped.
Mr Ainsworth held talks with Afghan President Hamid Karzai as well as senior British and US commanders during his trip to Afghanistan and visited the frontline at Sangin in Helmand.
He said the situation was "slightly more positive" than he had anticipated. There were "real signs of progress on the ground" in Helmand Province where UK troops are based.
"We should not be impatient about what can be achieved. There is still a long way to go in Afghanistan," he said.
He said the main thing was ensuring the Afghan national election to be held in August was "credible".
"At the end of the day it's all about safety for the people," he said.
"You don't win this by killing Taliban, you win this by providing a secure environment for ordinary Afghans, who are then able to get on with their lives."