Britain and Iraq announced an accelerated timetable for the withdrawal of foreign troops from much of Iraq today during a surprise visit by Tony Blair to show his support for the country's new government.

Britain could return two southern provinces to Iraqi security control within in a few months. Nouri al-Maliki, Iraq's new Prime Minister, said that he expected as many as 16 of the 18 provinces to be "Iraqi-ised" - under the control of Iraqi forces - by the end of this year.

The timetable will increase pressure on the United States to set its own deadlines for withdrawal, which Bush Administration officials have steadily refused to do for fear of handing a propaganda victory to al-Qaeda linked insurgents.

The Prime Minister landed in Baghdad's Green Zone in a Chinook Army helicopter after a hair-raising flight from Baghdad airport in which he skimmed low over the city to avoid attracting fire. He met US and British military commanders and members of the new national unity government, formed at the weekend after six months of wrangling.

Although a senior official accompanying him told reporters that the Multi-National Force in Iraq would wrap up its work in the next four years - the clearest deadline yet set by a Western official - Mr Blair refused to be drawn on the precise details of withdrawal.

In a joint press conference with Mr Blair, Mr al-Maliki was more forthcoming, saying that he expected as many as 16 of Iraq's 18 provinces to be under Iraqi security control by the end of this year.

"There is an agreement for the transfer of security under a timetable which starts in June when Iraqi forces will take control of the provinces of Samawa and Amara," Mr al-Maliki said.

"The other provinces will be transferred gradually and by the end of this year most of them will (be under Iraqi security control), with the exception of Baghdad and perhaps Al-Anbar" - referring to two provinces that have borne the brunt of insurgent violence.

After the press conference the two men released a joint communique spelling out once again the need for clear progress on transferring security control.

"The Iraqi Prime Minister said that his Government will, in the weeks ahead, work with the Multi-National Force (MNF) on the details of the transition to Iraqi control," the statement said.

"The Iraqi-isation of security means that, as security responsibility grows, so the MNF can over time reduce presence and focus instead on the role of helping train and develop the Iraqi security forces.

"The MNF, for its part, is committed to staying until the Iraqi government is satisfied that Iraqi forces can take on the security responsibilities themselves. The two governments agreed that they looked forward to Iraqi forces progressively and quickly taking on full responsibility for security from the multi-national forces in the cities and provinces of Iraq, on the basis of conditions set out by the Iraqi government to this end.

"The MNF, which is in Iraq under UN Resolution 1546, will stay only as long as the Iraqi government wishes it to. This process of transition will start in some provinces in the coming months, and by the end of this year responsibility for much of Iraq’s territorial security should have been transferred to Iraqi control."

It was Mr Blair's fifth visit to Iraq but only his second to the Iraqi capital and follows days of bloodshed surrounding the formation of the new Iraqi government - even though the key Interior and Defence portfolios have still be assigned. Dozens have been killed and injured in a string of suicide and roadside bombs and drive-by shootings.

The surprise visit also explained the Prime Minister's absence from Britain's celebrity event of the season last night - the pre-World Cup party hosted by David and Victoria Beckham at their mansion in Hertfordshire.

Mr Blair said: "It has been three years of struggle to try and get to this point. It has been longer and harder than any of us wanted it to be, but this is a new beginning.

"The important thing is that for the first time we have a government of national unity that crosses all boundaries and divides, that is there for a four year term and that it’s directly elected by the votes of millions of Iraqi people. There is now no vestige of an excuse for anyone to carry on with terrorism or bloodshed."

Britain has 7,200 troops in Iraq and is responsible for the security in four provinces, Basra, Dhi Qar, Maysan and Muthanna. The senior official accompanying Mr Blair said that it was hoped the last two could pass to Iraqi control by the summer.

The situation in Basra province, where most of the UK troops are based, makes withdrawal more problematic. The southern province is in the grip of growing lawlessness and largely run by Shia militias loyal to the radical cleric Moqtada al-Sadr.

Mr Blair is due to visit Washington later this week for discussions with President Bush on future strategy in Iraq - talks which could be less amicable than normal if Britain is indeed trying to force Mr Bush's hand on withdrawals. The US has 130,000 troops on the ground in Iraq but refuses to set any firm deadlines for their departure.

Mr Blair said: "We have always, under the Iraqi-isation strategy, had the perspective of building up the Iraqi security force capability and as they build up, we are able to draw down.

"That is what was envisaged in the UN resolution under which our forces are here. We want to move as fast as we can on it, but it has got to be done in a way that protects the security of the Iraqi people."

He added: "We have got the Iraqi forces up to a strength, I think, of round about 250,000. They come up to their full strength at the end of this year.

"There is the notion, which has been there for a significant period of time, of slowly being able to release individual provinces into the control of Iraqi forces. But, as we both stress, it is an objective timetable in the sense that it depends on the conditions on the ground."

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