Mr Blair said Britain was "ready to play its part"
Speaking in Downing Street following talks with US Secretary of State Colin Powell, Mr Blair said decisions on the deployment of a force would have to be made "relatively quickly".
But he said there was still "an immense amount of details to be decided and discussions to be had" before any force was put in place.
Mr Blair said it was always clear that once the military campaign in Afghanistan had achieved its objectives new operations would have to be put in place to ensure security and aid efforts.
But he said: "There are a whole host of things to be discussed and bolted down before we are in a position to do so."
He said the coalition's actions since 11 September had sent a very clear message to terrorists.
Extending war
But he also warned that although the military campaign in Afghanistan had gone well, al-Qaeda had to be beaten wherever it operated.
Mr Blair said: "We have concentrated on achieving our objectives in Afghanistan. Of course the battle against terrorism doesn't end there.
"Of course Britain stands ready to play its part in that."
Mr Powell said US President George Bush had not yet taken any decisions on the direction of "phase two" of the war against terrorism.
But he emphasised that Osama Bin Laden's al-Qaeda network had to be destroyed wherever it operated.
UK Foreign Secretary Jack Straw said later: "We believe we are now in the closing stages of seeing a proper end to the al-Qaeda organisation and those who lead them."
Mr Straw branded al-Qaeda's leaders cowards.
"They have been very ready to sacrifice other people's lives - but they are not willing apparently to sacrifice their own lives or give themselves up to justice," he said.
European support
"I'm quite clear that, one way or another, they will be brought to justice or justice will be brought to them."
Before his meeting Mr Blair in London, Mr Powell travelled to Paris to meet French President Jacques Chirac and French Foreign Minister Hubert Vedrine.
Mr Vedrine said France would play "an active role" in the effort to rebuild Afghanistan and maintain peace there.
German Chancellor Gerhard Schroeder also reiterated on Monday during a visit by Mr Powell that Germany would participate in a force if asked.