Bush's new jab in a tussle over ending the unpopular conflict, came as Democratic leaders mustered a fresh challenge to his strategy, which would also see a vote on whether to choke off war funding for good in July.

The House of Representatives bill, a response to Bush's veto last week of a 124 billion dollar war budget which included Democratic timelines for withdrawal from Iraq, could come to a vote as early as Thursday.

Asked on Air Force One as Bush headed to Kansas to view tornado damage whether the president would veto the new bill if it reached his desk, White House spokesman Tony Snow replied "Yes."

US Defense Secretary Robert Gates meanwhile told a Senate committee a vote to cut funds for the war in July would cripple the Pentagon.

"A 'no' vote in July would have dramatic consequences," Gates said.

"We will have forward spent so much money to keep the troops in the field by that time that the truth is ... I would have to shut down significant elements of the Department of Defense in August and September because I wouldn't have the money to pay salaries."

Republicans in Congress meanwhile issued a fierce counter-attack against the Democratic plan unveiled Tuesday, saying it was to an open invitation for insurgents to kill as many Americans in as short a time as possible.

Republican House whip Roy Blunt branded the measure "dumb and dangerous" and accused his rivals of engaging in Act Two of a melodrama over the fate of the US involvement in Iraq.

"If we enact this bill today, you put the insurgents and extremists on an 83 day time clock ... see how many young Americans you can kill in 83 days," Blunt said.

"This is a worse plan than the terrible plan they had before," he said referring to the vetoed Democratic targets to bring most US combat troops by home by March 2008.

Democrats are striving to find a way to satisfy their restive anti-war support base and further their drive to bring troops home, yet find a bill on funding troops in the field that the president will sign.

But White House counselor Dan Bartlett accused the Democrats of indulging in political games by framing a plan they knew Bush would not accept.

"Instead of sitting around the table and negotiating a bill that we can all accept they've chosen to put forward legislation (...) that we cannot accept," he said.

It was still uncertain if Bush would ever have to wield his veto as chances of a similar split funding bill passing through the more closely divided Senate appeared dim, and composite legislation must be agreed between both chambers before being sent to the president.

The new Democratic plan would offer 40 billion dollars to fund the wars in Iraq and Afghanistan only through the summer months, and withhold a further 50 billion dollars pending a report by Bush on progress towards a series of political, security and economic benchmarks in Iraq.

Congress would then vote on whether to release remaining funds. Should lawmakers vote to cut funding, troop withdrawals would have to begin within 180 days.

As the fierce wrangle over Iraq intensified, Vice President Dick Cheney arrived in Baghdad on an unannounced visit, and said he was confident the Iraq government understood the urgent need for progress.

Since winning control of Congress last November, Democrats have subjected Bush's Iraq war strategy to unprecedented pressure, and are trying to turn up the heat on Republican lawmakers with elections looming in 2008.

Republican Senator Gordon Smith, one of only two of his party's senators to vote for withdrawal timelines warned Wednesday dissent was mounting over Iraq among Bush backers.

"I hear from many Republican Senators ... many of them will simply change their votes and Chuck Hagel and I will not be the only ones calling on the president to put the troops in a new place," he told NBC.

September is emerging as a key date in the congressional showdown with Bush, with General David Petraeus expected to report back on the progress of the president's surge strategy.

Several key Republicans have said in recent days that they will need to see progress in the strategy to surge nearly 30,000 more troops into Iraq, or expect the administration to come up with a "Plan B."