The trickle of Republican rebellion against President George W.Bush's Iraq policy has turned into a stream with two more previously loyal senators joining the swelling ranks ofcritics.

Lamar Alexander said "it should be clear to the President that there needs to be a new strategy", while Judd Gregg called for a "clear blueprint for how we were going to draw down".

Although they stopped short of backing the Democratic proposals for a fixed timetable of withdrawing US troops from Iraq, their comments came before a week in which the Senate is once again debating funding for the war and the military will deliver an interim progress report on Mr Bush's "surge strategy".

The interim report is believed to conclude that US combat deaths have escalated, violence has spread beyond Baghdad and sectarianism has further polarised Iraq, the Washington Post has reported.

It is also expected to acknowledge tacitly that the Iraqi Government is unlikely to meet any of the political and security goals Mr Bush set for it in January.

"The security progress we're making in Iraq is real," a senior intelligence official in Baghdad was quoted as saying, "but it's only in part of the country and there's not enough political progress to get us over the line in September."

Six Republican senators have now announced they can no longer support Mr Bush's Iraq strategy, and demanded change.

The dissent has been led by senior figures including Richard Lugar and John Warner, the Republican leaders - or "ranking members" - on the Senate foreign and armed forces committees.

Senator Lugar yesterday used a TV interview to explain that his public intervention had been intended as a "reaching-out to the President".

He suggested that remaining opportunities for a centrist "bipartisan consensus" on Iraq were fast disappearing before presidential elections next year in which Democrats are being pushed by activists to call for an immediate withdrawal of troops.

Along with other Republicans who have spoken out in recent days, such as George Voinovich and Pete Domenici, Senator Lugar appears to be advocating measures similar to those of last year's Iraq Study Group report.

That report proposed shifting American troops away from the sectarian conflict between Sunnis and Shi'ites so that a reduced force could concentrate on counter-terrorism and support functions.

"We have to be thoughtful about the safest route for our forces out of Iraq," Senator Lugar said, adding that most ofthe 160,000 US troops stationed in the country could be "redeployed by the middle part of next year".

Moderate Republicans fear that Mr Bush's apparent determination to fight on will lead to a panic pullout that could set off awider conflict across the MiddleEast.

But the White House is urging them to hold the line at least until September, when General David Petraeus will present his assessment of the effectiveness of the 30,000-troop "surge".

Mr Bush has repeatedly said he wants as much time as possible for his 30,000-troop increase to show results.

But the American military is already overstretched, with the Pentagon making troops serve longer in battle zones - and more often - as it tries to sustain the surge.

There is now growing pressure on Mr Bush to pre-empt the September report by setting out a time frame for withdrawing at least some of the troops.

Some Bush aides believe forces are combining against him as the Senate prepares to begin a contentious debate on the defence authorisation bill.

Democrats are gearing up for a new push for a US exit from Iraq amid a surge in bloodshed at the weekend.

With public discontent with the war growing, house Speaker Nancy Pelosi is set to introduce a bill to authorise troop redeployments to start within four months and be completed by April, a formula Mr Bush has blocked once with a presidential veto.

But aides are now believed to be advising Mr Bush that if he wants to forestall more defections, it would be wiser to announce plans for a more narrowly defined mission for US troops that would allow for a staged pullback, The New York Times reported yesterday.

The number of US combat brigades in Iraq is, in any case, scheduled to be reduced from 20 to 15 by this time next year.

Meanwhile, US military sources in Baghdad said fiery Iraqi Shia cleric Moqtada al-Sadr, head of the the Mahdi Army militia insurgency group, has gone back to Iran. Sadr led two uprisings against US forces in 2004 before becoming more involved in mainstream politics.