The Congressional Budget Office warned that if the current policy of keeping army units in a war zone no longer than one year is preserved, the US Army "would be unable to sustain" its present Iraq contingent "beyond about March 2004."

The report was certain to add fuel to the growing debate in Washington over the Bush administration's readiness to handle post-Saddam Iraq, including the costs of occupation.

Senior US officials said Tuesday the United States would seek a new UN Security Council resolution to expand the United Nations mandate in Iraq, making it easier for more countries to participate in the stabilization force.

The enormous expense -- coupled with the estimated four billion dollars a month being spent on the military occupation of Iraq, growing criticism of the operation and a rising death toll among US troops -- has created a new urgency in recruiting more countries to join the stabilization effort.

More than 180,000 US troops are currently deployed in Iraq and neighboring Gulf nations in support of Operation Iraqi Freedom, according to congressional officials.

But soldiers hunting down remnants of the Saddam Hussein regime will have to start withdrawing next spring, and the US Army lacks fresh troops to replace them all, the officials said.

As a result, the Army will be able to keep in Iraq indefinitely only between 38,000 and 64,000 troops, if it relies exclusively on its active duty soldiers and some reserve units, according to the CBO report, advance copies of which circulated on Capitol Hill late Tuesday.

Maintaining such an occupation force would cost the US taxpayer up to 12 billion dollars a year.

To overcome the shortage, the Pentagon could marshal the resources of the Marine Corps, various special operations units and the National Guard and bring the sustainable occupation force to up to 106,000 military personnel.

But that would boost occupation costs to up to 19 billion dollars a year, the report said.