In their latest attempt to tie President George W Bush's hands on the war, Democrats unveiled a new amendment to a Senate defence policy bill that would subject the US mission in Iraq to strict limits.

The bill, co-sponsored by Senator Carl Levin, chairman of the Senate Armed Services committee and fellow Democrat Jack Reed, would curtail operations to fighting terrorism, training Iraqi troops and protecting US assets.

It would effectively end Bush's strategy to surge up to 30,000 extra troops into Iraq, and is designed to exploit rising concern among Bush's Republican backers over developments in Iraq.

Bush has previously vetoed a Democratic attempt to force him to accept troop withdrawal timetables, but congressional leaders are trying to piece together a veto-proof two-thirds majority for the new bill.

"President Bush had a pre-surge strategy, and a surge strategy and now apparently he is going to offer a post surge strategy later this week," said Levin.

"All of these strategies are marked by one thing -- they all have an open-ended commitment with the US forces in the middle of sectarian violence."

The measure calls for the reduction of US troops to start no later than 120 days after the enactment of the law.

It states "The Secretary of Defense shall complete the transition of United States forces to a limited presence and missions ... by April 30, 2008."