Mr Hutton said more security was needed in Afghanistan
Further increases in UK troop levels in Afghanistan cannot be ruled out, Defence Secretary John Hutton has said.
Britain would consider "very carefully" any request from US President-elect Barack Obama for more forces, he said.
Last week Gordon Brown said the number of UK troops was to rise by 300 in March, and up to 30,000 new US troops are expected to be sent in early 2009.
Mr Hutton told Sky News the UK provided the second largest contribution to Nato forces in Afghanistan.
Mr Obama is thought likely to ask Nato allies to increase their contributions.
Speaking on Sky News' Sunday Live, Mr Hutton said: "I'm not going to speculate today about future force levels, because we act on the advice we receive from service chiefs and our friends and allies around the world.
"As the prime minister has said, of course we would look at any request for additional forces seriously."
He said no such request had been received from the US, adding: "When we receive such a request obviously we would look at it very, very carefully."
He said it was "clear" more security was needed in Afghanistan, but insisted international forces were "making headway" in the country.
"I do think we should be prepared to see this mission through, because first and foremost it is about UK national security. We can't allow the terrorists to get hold of Afghanistan again because we know what would happen.
"They would have a safe haven to launch their terrorist attacks against us."
Shadow foreign secretary William Hague told BBC1's Andrew Marr Show that he agreed sending more troops should not be ruled out.
But he added: "We have got to bear in mind that the British troops have borne a disproportionate share of the burden so far and many other Nato nations have not engaged in the serious fighting British troops have had to do.
"For more British troops to go we would expect there to be a revised and clear and effective strategy in Afghanistan," he said.
Last week the prime minister said the 300-strong rise in numbers would take the total UK deployment to 8,300 from March until August next year, with the extra soldiers carrying out "security" operations.
In his statement, Mr Brown called on some of Britain's allies to contribute more "fairly" to military efforts.