WASHINGTON, Sept 8 (Reuters) - U.S. President George W. Bush could announce as soon as Tuesday that he wants to withdraw one combat brigade of about 4,000 troops from Iraq early next year, leaving force levels largely unchanged until he leaves office.

White House spokeswoman Dana Perino would not comment on the substance of Bush's plan but said it would likely be announced this week.

"I think it's probable that you'll hear it this week, possibly as early as tomorrow -- I should say likely, even probable tomorrow," Perino said on Monday.

Top U.S. defense officials recommended the plan to Bush last week. Pentagon sources have said the plan includes withdrawing one combat brigade from Iraq early next year.

Bush, who has seen his job approval rating sink because of the prolonged war, could to make his announcement when he speaks at the National Defense University in Washington at 9:55 a.m. EDT (1355 GMT) on Tuesday.

There are 146,000 troops in Iraq and 33,000 in Afghanistan. U.S. Army combat brigades in Iraq typically have around 3,500 to 4,000 soldiers, according to defense officials.

Three Pentagon sources told Reuters last week that Army Gen. David Petraeus, the top commander in Iraq, had agreed to shift from 15 brigades to 14 in that war zone. Two sources said the change would not take place until early next year.

Additionally, according to a senior U.S. defense official, one Marine battalion would be withdrawn from Iraq and its replacement would go this November to Afghanistan where Taliban and al Qaeda militant attacks on U.S.-led coalition forces have increased over two years.

While violence in Iraq has dropped dramatically in the past year, the proposed cutback is smaller than some analysts had predicted, reflecting caution on the part of Petraeus, who is anxious not to jeopardize security gains.

Already, the Pentagon has withdrawn five combat brigades from Iraq this year after a "surge" strategy that added more than 30,000 troops to Iraq.

The White House on Monday defended that surge strategydespite the reported resistance from top military leaders.

U.S. journalist Bob Woodward wrote in a new book titled "The War Within: A Secret White House History 2006-2008," that some members of the Joint Chiefs of Staff and commanders in Iraq disagreed with sending more troops to the war.

"I think that the surge is no doubt one of the most important foreign policy and military decisions that have been made in a generation, and it was fundamental to the change that we have seen today in Iraq," Perino said.

"We are working now to cement those gains and to be able to continue to watch Iraq evolve into a country that can sustain, govern and defend itself," she said. (Reporting by Jeremy Pelofsky and Andrew Gray, editing by Kristin Roberts and Jackie Frank)

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