The military campaign in Afghanistan is "winnable", Liberal Democrat leader Sir Menzies Campbell has said.
UK troop numbers in Afghanistan are due to be increased
Speaking on BBC's Sunday AM, he said it was a difficult situation, but the country could gain stability.
He spoke after news that Britain's 5,600 troops in the country were due to be boosted, possibly by 1,000.
Deployment in Afghanistan had had a "clear set of political objectives," but resources were needed to fulfil military objectives, Sir Menzies said.
The British troops are part of Nato's International Security Assistance Force.
Defence Secretary Des Browne confirmed last week more troops would be sent, but added a statement would be made in the Commons on Monday.
The increase comes days after Prime Minister Tony Blair announced troops in Iraq would be reduced this year by 1,600.
Sir Menzies said Afghanistan was difficult, with "ferocious" fighting, and that some people said it is was dangerous as the Korean War.
"But there is no doubt that this is in a different category altogether from Iraq and it is somewhere where we should be putting resources to bring about, as far as we can, a successful conclusion," he said.
He added: "I think it is winnable: that's the judgement of the senior commanders.
"But there's no doubt that there is a clear set of political objectives. What we need are clear military objectives but also, of course, fundamentally we need adequate resources so we can achieve both these military and political objectives".
After Mr Browne confirmed more troops would be sent to Afghanistan, the Tories said it showed British forces were too "overstretched" to carry out duties in both there and in Iraq.
Britain has recently revamped its operations in Afghanistan to put most manpower into Helmand province in the south, where the fighting is at its most fierce.
The 1,300 troops currently in Kabul will come out of that region shortly.
The majority of those will go south to Helmand, except for about 400 who will leave Afghanistan.
The remaining 5,200 troops in the country will be bolstered by the expected extra 1,000 troops, making the UK force in Afghanistan 6,200-strong.