The US Senate is due to vote on three major Iraq war proposals

Senators are due to vote on three major amendments to a defence funding bill, one of which would require withdrawal of US troops by the end of April 2008.

The Senate's Democratic leader has said a rare all-night sitting will be held on Tuesday before a vote on Wednesday.

US President George W Bush has insisted his "surge" strategy must be given time to work before changing course in Iraq.

The pressure on Mr Bush is mounting, with some Republicans withdrawing support and an interim progress report last week which showed mixed results in Iraq.

Mr Bush has said Congress should wait until a 15 September progress report on the surge strategy, under which some 30,000 extra troops have been deployed, before considering a change in direction.

Veto threat

Democratic Senate Majority Leader Harry Reid has scheduled Tuesday's unusual round-the-clock sitting in a bid to squeeze Republicans who continue to support Mr Bush's war policy.

President Bush says time must be given for the surge strategy to work

The Democrats are seeking to pass legislation that would require the withdrawal of most US troops from Iraq by the end of April 2008, with redeployment starting within 120 days of the bill's passage.

But they are unlikely to succeed in Wednesday's vote because the Republicans are using a procedural measure to insist on 60 votes for victory rather than a simple majority of the Senate's 100 members.

Even if passed, Mr Bush has already threatened to veto the measure. The Democrats would need a two-thirds majority in House of Representatives and Senate to overturn a presidential veto.

The House approved a similar bill that would require the withdrawal of US combat troops from Iraq by 1 April 2008 last week.

Bipartisan plan

A vote is also expected this week on a blueprint put forward by senior Republican Senators Richard Lugar and John Warner which urges Mr Bush to begin pulling US troops out of Iraq by the end of the year.

They want Mr Bush to submit plans by 16 October for the "transition of US combat forces from policing the civil strife or sectarian violence in Iraq" to more narrowly defined goals of tackling terrorism, guarding borders and protecting assets and coalition forces.

The White House and Democrats have already reacted coolly to the Lugar-Warner proposal.

Senator Reid criticised the policy because it did not insist on any implementation or require troop withdrawal by a certain date.

A third, bipartisan, proposal to be voted on this week would have the US adopt as policy recommendations made last December by the Iraq Study Group (ISG).

The ISG report called for the redeployment of US forces from combat to training and other roles, and for the US to reach out diplomatically to Iraq's neighbours, including Iran and Syria.

Last week's Iraqi interim report highlighted among other issues a lack of progress in training Iraqi security forces.

The number of Iraqi battalions ready and able to fight on their own has halved in recent months, despite increased efforts by the US to train them.