The Democrats wanted US troops to have time off between tours in Iraq equal to their 15-month deployments.

The measure needed 60 votes to pass in the Democratic-controlled Senate but received only 56 votes with 44 against.

US Defence Secretary Robert Gates called it a backdoor attempt to pull troops off the battlefield. He warned that he would have recommended US President George W Bush veto the measure had it passed.

Republican Senators who were dubious over the president's policy are now giving him more time after an upbeat assessment last week of progress in Iraq by the top US commander in Iraq, Gen David Petraeus.

Supporters of the bill, which was also narrowly defeated in July, said opponents were ignoring the troops' interests.

"In blocking this bipartisan bill, Republicans have once again demonstrated that they are more committed to protecting the president than protecting our troops," Senate Majority leader Harry Reid said. Senator Chuck Hagel, one of six Republicans who backed the bill, said: "We cannot continue to look at war and the people who fight and die in wars as abstractions, as pawns, as objects."

But critics said it would have interfered with Mr Bush's plans to reduce US troop levels in Iraq by around 30,000 to about 130,000 by mid-2008.

US troops currently have about 12 months rest at home between deployments.