Britain's Prime Minister Tony Blair leaves Downing Street for Prime Minister's Questions in the House of Commons, London, February 21, 2007. Blair will on Wednesday announce a timetable for the withdrawal of British troops from Iraq just as thousands of additional U.S. troops are arriving there to try to restore order in Baghdad. [Reuters]

Blair telephoned the US president on Tuesday to warn him of the announcement, the White House said.

"President Bush sees this as a sign of success and what is possible for us once we help the Iraqis deal with the sectarian violence in Baghdad," said White House spokesman Gordon Johndroe.

Vice President Dick Cheney said the British drawdown was justified given the situation in Basra, but stressed there could be no retreat from Iraq.

"I want you to know that the American people will not support a policy of retreat," Cheney said on the USS Kitty Hawk aircraft carrier while on a trip to Japan.

"We want to complete the mission, we want to get it done right, and we want to return with honour," he said.

The withdrawal represented not only a political blow to the US administration from one of its staunchest allies, it also carried the risk of plunging another part of Iraq into violent chaos, some analysts warned.

Although relatively calm compared with Baghdad and Sunni areas to the north and west of the capital, Basra has been the scene of spurts of internecine violence among rival Shiite militias.

Nora Bensahel, a terrorism expert at the US-based Rand Corp, a think tank with ties to the US Air Force, said that while violence in the south is at a manageable level, she expected the security situation to worsen when British soldiers leave.

"There certainly is no more slack in the US system to deploy more troops to the south were that seen to be necessary," she said.

"So if the level of violence does go up in the aftermath of the British withdrawal -- and I think that is a very likely scenario -- that could be very problematic for the US," she said.

Pentagon officials said the British withdrawal in the coming months was unlikely to have an effect on the day-to-day operations of the US military, which runs its supplies through the oil-rich southern region.

But other US analysts said the British troops are badly needed, but in Baghdad where sectarian violence is mounting.

Australian Prime Minister John Howard, another strong backer of the US-led invasion, said he would not reduce Australia's 550 combat troops in Iraq, part of a 1,400-strong force in the country.

At the same time, Bulgaria's parliament voted to extend the presence of the 155 Bulgarian troops in Iraq by a year until April 2008.

And El Salvador -- the only Latin American to still maintain troops in Iraq -- planned to keep its 380 troops in the country, a spokesman for President Elias Antonio Saca said Wednesday.

The United States has now more than 140,000 troops in Iraq in the wake of Bush's January announcement that he would send another 21,500 to Baghdad and surrounding provinces where the insurgency and sectarian violence have worsened in recent months.

There was more carnage in Iraq on Wednesday.

A suicide car bomber attacked an Iraqi police checkpoint in the Shiite holy city of Najaf, killing at least 13 people, and three more US troops were killed in two separate incidents.

Iraqi officials welcomed news of Britain's withdrawal plans. Sami al-Askari, a Shiite lawmaker and political adviser to Maliki, said it fulfilled "the wish of the Iraqi government and all the political powers in the country."

Hakim al-Mayahi, head of the Basra provincial security council which has had a fractious relationship with the British, said he welcomed "any withdrawal of British forces from inside the centre of the city."

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