The US plan to send 20,000 extra troops to Iraq is "entirely consistent" with the UK's aim of removing "thousands" of soldiers, the defence secretary says.
The vast majority of the new US troops will be based in Baghdad
But Des Browne declined to speculate on any timetable for withdrawal.
Foreign Secretary Margaret Beckett played down the likely impact of America's decision to send in more troops, mainly to the Baghdad area.
The effect on British soldiers - mainly based around Basra - would be "somewhat limited", she told MPs.
Mr Browne said the UK would reduce troop numbers by "thousands" in the coming months, but declined to give any detailed numbers, to protect the "operational security of those who are deployed".
"As soon as I am in a position to say to Parliament what we are doing, then I will," he told a special joint session of the commons defence committee and the foreign affairs committee.
Mrs Beckett told the MPs there had been no "split" over tactics.
She said she fully supported US President George Bush's decision.
"Obviously it's an issue that people will look at but I would say that it's a change of direction, as the president says, for the United States and doesn't necessarily imply a change of direction for us," Mrs Beckett added.
She said there was "not a fundamental difference in our approach".
Mrs Beckett earlier dismissed press reports that 3,000 British troops would be brought home by May as "speculation".
Prime Minister Tony Blair also rejected any suggestion Britain was at odds with US policy.
He told West Country ITV: "It is really important that we don't either give that impression or have that misunderstanding
"The conditions in Baghdad are very different from those in Basra.
"In Basra we don't have the same threat from al-Qaeda, we don't have the same sectarian violence to anything like the same degree, and we don't have the same insurgency."
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.
Shadow Foreign Secretary William Hague said he was "sceptical" about the US plan and that he would prefer more emphasis on training and equipping Iraqi forces.