The Bush administration has avoided predicting how long it will keep the beefed-up force of about 160,000 troops ordered by the president in January.

It has said only that it will review progress in the late summer, a stance stressed last week by Defense Secretary Robert Gates when he visited Iraq. The implication is that troops could then start to be withdrawn but that appears improbable.

Army Gen. David Petraeus, the top U.S. commander in Iraq, said during Gates' visit that the buildup of some 28,000 extra troops would not even be complete for another two months.

"It'll be in mid-June or so when all of the forces will be in place and we can see the effects of the real surge and even then it will take some time, we think, before you can actually develop what that capability will provide you," he said.

U.S. commanders have said that the latest security crackdown, launched to coincide with the buildup of troops, has yielded mixed results so far, with a drop in sectarian murders but a rise in high-casualty car bomb attacks.

Petraeus has cautioned that achieving success with the extra troops will take months, which suggests maintaining the higher level for more than just the 10 weeks or so between reaching full strength and an initial progress review.

Bush administration officials have stressed that the new military approach goes hand in hand with an increased effort to boost reconstruction and economic development.

But provincial reconstruction teams of U.S. experts assigned to help strengthen Iraqi institutions -- a key element of the plan -- are not due to reach full strength until the end of this year, said Rick Olson, provincial reconstruction team coordinator.

"My sense is that this civilian surge and the duration thereof will be tied to the military surge," Olson, a retired major general, told reporters traveling with Gates.

"We take Gen. Petraeus at his word that this is not something that's going to be done overnight. This is going to be a longer-term effort," he said.

"I would say that from my vantage point we will see however many brigades Gen. Petraeus has asked for. We'll see that many brigades here in December."

The current troop increase approved by Petraeus will put 20 U.S. combat brigades in Iraq.

TOURS EXTENDED

The Pentagon has also extended the tours of duty of U.S. soldiers in Iraq from a year to 15 months, a measure Gates said would allow the United States to maintain the higher troop level in Iraq until next spring.

But any mention of a specific end-date for the increased force level is politically sensitive.

President George W. Bush's Republican administration is under unrelenting pressure from Democrats, who won control of Congress last November largely due to voter anger over the war and believe they have a mandate to push for withdrawal.

Any suggestion that the buildup that began in January may last until the year's end or beyond risks fueling domestic opposition to a war in which more than 3,300 American soldiers and at least tens of thousands of Iraqis have died.

Even so, military commanders are unwilling to telegraph a date when higher force levels will be reduced as they believe that would encourage their enemies to wait them out and then resume violence.

Gates' visit to Iraq showed Washington hopes to use uncertainty about the end-date to push Iraq's Shi'ites, Sunnis and Kurds to adopt reconciliation measures quickly.

Gates said the troop increase was meant to buy Iraqi leaders time to reconcile and their record on that front would be a factor in his review of force levels in the late summer.

But he acknowledged the plan was not expected to provide instant success. "We do need some time to try and make this work," he said.