Levin said the Iraq war resolution approved by the Senate more than four years ago "was a wide open authorisation which allowed him [Bush] to do just about anything and put us now deep into combat in Iraq, and now into the neighborhoods of Baghdad.

"I think we'll be looking at a modification of that authorisation in order to limit the mission of American troops to a support mission instead of a combat mission, and that is very different from cutting off funds," he said.

Authority challenged

Joseph Biden, senate foreign relations committee chairman, says there is a need "to repeal and restate the president's authority" to ensure US troops were not drawn into an escalating sectarian conflict.

Before the senate resolution was blocked, a non-binding resolution criticising Bush's war plan had passed Friday in the house of representatives, by a 246-182 vote.

Sixty votes were needed in the 100-seat senate to move to a vote on a resolution condemning the administration's plan to commit extra troops to Iraq.

However on Saturday, only 56 senators, including seven Republicans, voted to allow debate to start, four votes short of the minimum requirement.

The Bush administration said on Sunday that congress should support upcoming spending bills for US troops in Iraq, given that the votes in both houses had concluded.

"What we are saying is, if you want to support the troops, give them the reinforcements they need to succeed in the new and different kind of mission," Tony Snow, White House spokesman, said.