President George W Bush views the plan to cut British forces to about 5,000 by the end of the summer as "a sign of success", according to a US National Security Council spokesman, Gordon Johndroe.

Mr Bush spoke to the prime minister about the plans by video link yesterday, Mr Johndroe said.

"While the United Kingdom is maintaining a robust force in southern Iraq, we're pleased that conditions in Basra have improved sufficiently that they are able to transition more control to the Iraqis," he said.

Vice president Dick Cheney, on a visit to Japan, endorsed that view.

"I look at it and what I see is an affirmation of the fact that in parts of Iraq ... things are going pretty well," he told ABC News.

Mr Cheney said a friend who had just travelled from Baghdad to Basra, where most British troops are based, "found the situation dramatically improved compared to where it was a year or so ago".

However Mr Cheney said Washington remained committed to its own plan to send another 21,500 troops to Iraq to try to stop violence in Baghdad and other areas.

He condemned opposition to the US plan by Nancy Pelosi, the speaker of the US House of Representatives, and Representative John Murtha, both Democrats.

"I think in fact if we were to do what Speaker Pelosi and Congressman Murtha are suggesting, all we'll do is validate the al-Qaida strategy," Mr Cheney said. "The al-Qaida strategy is to break the will of the American people."

The US secretary of state, Condoleezza Rice, said "the coalition remains intact", despite Britain's move.

"The British have done what is really the plan for the country as a whole, which is to transfer security responsibility to the Iraqis as the situation permits," she said after a meeting in Berlin with the German foreign minister, Frank-Walter Steinmeier.

"The coalition remains intact and, in fact, the British still have thousands of troops deployed in Iraq."