Foreign Secretary Margaret Beckett has welcomed US plans to send more than 20,000 troops to Iraq, but said the UK has no plans to do the same.
The vast majority of the new US troops will be based in Baghdad
She said UK troops were successfully quelling violence in British-controlled Basra, and there was no intention "at the present time" to send more.
Reports 3,000 troops would leave Iraq by May were "speculation," she added.
The Tories said they were "sceptical" about the US plans and the Lib Dems argued it might make things worse.
Lib Dem leader Sir Menzies Campbell called for a phased withdrawal of troops, saying Britain "should not reinforce failure" by remaining in Iraq.
The plans were a "last-ditch...go-it-alone" effort, said Sir Menzies, as US President George Bush had apparently rejected suggestions he should engage with Iran and Syria to help stabilise Iraq.
On Wednesday, Mr Bush announced extra US troops to fight alongside Iraqi units to end violence in Baghdad and Anbar province - where he said al-Qaeda terrorists were planning to take control.
But the US commitment to Iraq was "not open-ended", he said, adding that he expected the Baghdad government to fulfil its own promises.
Mrs Beckett said it showed both President Bush and Iraqi Prime Minister Nouri Maliki were "determined to try to come to grips with what is unquestionably a difficult situation in, particularly in Baghdad.
"We welcome that and we hope that the joint effort to resolve this very difficult security situation which is undermining efforts to put other things right in Iraq will indeed succeed."
But she said UK forces were already engaged in a similar operation in the southern city of Basra.
"We are dealing with the security situation in Basra. It's not our intention at the present time to send more troops," she said.
She said she was "not aware" of any suggestions British troops might be redeployed to help secure Baghdad.
And she said a Daily Telegraph report that there were plans to send nearly 3,000 British troops home by the end of May was "speculation".
Mrs Beckett welcomed the announcement
"We are under way with the process of handover as the security situation improves.
"We will make our judgements and our decisions depending on the progress of those events...The Telegraph may speculate about timing and so on, but it does depend on how things go in Basra."
Shadow Foreign Secretary William Hague said he was not convinced about the US plan, saying previous attempts to secure Baghdad that way had failed.
He told the BBC the long-term presence of US troops in Iraq had fuelled the insurgency, and he would prefer more emphasis on training up and equipping Iraqi forces to take on more security duties.
He added: "I'm sceptical about the policy and the impact of sending more American troops to Iraq."
Conservative leader David Cameron, on a visit to Northamptonshire, said: "I think there are some encouraging things in what the President has said in terms of more control for the Iraqi authorities but I'm sceptical about the idea of surging troop numbers.
"But that's an issue for the Americans. Generally, I'd like to see an approach that's closer to the Baker-Hamilton report."
Sir Jeremy Greenstock, the former British ambassador to the UN, was asked what would happen if increasing the number of US troops in Iraq did not quell the violence.
He said: "Then there's a case for saying the coalition can't do any more good and there's still a case for British withdrawal."
From 1530 GMT on Thursday Mrs Beckett and Defence Secretary Des Browne will be grilled on Iraq by a special joint session of the Defence and Foreign Affairs committees.
And Commons leader Jack Straw announced there would be a debate next Wednesday on Iraq and the Middle East.