A key Senate vote on Iraq is due next week

Richard Lugar and John Warner said sectarian violence could not be stopped "any time soon" and "probably cannot be controlled from the top".

They want Mr Bush to submit "transition" plans by 16 October but have not set a date for withdrawal.

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

Their proposal came a day after the Iraqi interim report which 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.

Tackling terrorism

Senators Lugar and Warner said they wanted to ensure US policy was prepared for change when the long-awaited report by US commander in Iraq, Gen David Petraeus, is published in September.

Their new blueprint urges Mr Bush to begin troop withdrawal by the end of the year.

It calls on him to submit a plan 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.

But the Democratic Senate Majority Leader Harry Reid said the plan did not insist on any implementation. He backed legislation to be set before the Senate next week that would require troops to leave by spring next year.

"If you give this president a choice, he will stay hunkered down in Iraq for years to come," Mr Reid said.

A spokesman for Mr Bush, Tony Fratto, said the White House would look at the new proposal but believed the security surge under way in Iraq "deserves the time to succeed".

Analysts say the Lugar-Warner proposal may draw support from Republicans who have expressed concern over Iraq but are not prepared to support Democratic measures.