Republicans have blocked a debate on the Iraq war in the US Senate, dealing a setback to critics of George Bush's plan to send 21,500 more troops to Iraq.

Republicans largely united to employ Senate rules against the Democrat majority to derail the debate on a nonbinding resolution expressing disagreement with Bush's plan.

But Democrats vowed that they would not give up trying to force Bush to change course and said they would return to the subject when considering billions more in funding for the Iraq war requested by Bush on Monday.

Harry Reid, the Senate Democrat leader, said: "We are going to debate Iraq.

The resolution would not have been binding on Bush, but it was the first serious attempt by Congress to confront him over the war.

Under Senate rules it needed 60 votes before the 100-member Senate could begin debate. It received only 49, with 47 voting against in a largely party-line vote.

Opponents said the measure, sponsored by Virginia Republican senator John Warner and Michigan Democrat senator Carl Levin, was a thinly disguised political slap at Bush that would dishearten US troops and signal American disunity.

Republicans also said they voted against the measure in protest because they could not get amendments considered on their terms.

Supporters said the resolution would be a first step, a warning to Bush that he must revamp his strategy to start moving toward a withdrawal of the 138,000 US troops in Iraq.

Some Democrats want Congress to move immediately to do something more decisive like refusing to fund the additional troops or capping troop levels.

Dana Perino, a White House spokeswoman, said: "All sides have a right to be heard in this debate, and we support senator McConnell's and the Republicans' right to be able to offer the amendments they want to offer."

Two Republicans refused to follow their party leadership and voted with the Democrats to move to debate. They were senator Susan Collins of Maine and senator Norm Coleman of Minnesota, both of whom face re-election in 2008.

PHOTO CAPTION

US Republican Senator Chuck Hagel from Nebraska speaks in Washington in January 2007. (AFP)