WASHINGTON  Security in Iraq continues to improve even after the withdrawal of nearly 25% of U.S. combat brigades, increasing the prospects of further cuts in American forces.

Although U.S. commanders are cautious about predicting further withdrawals, interviews with military experts and recent official statements indicate growing optimism about the potential to pull out more forces.

"I believe the momentum we have is not reversible," said Jack Keane, a retired Army vice chief of staff who helped develop the Iraq strategy adopted by President Bush in January 2007.

There will be "significant reductions in 2009 whoever becomes president," said Keane, who regularly consults with Gen. David Petraeus, the top U.S. commander in Iraq.

Iraqi Prime Minister Nouri al-Maliki echoed Keane's optimism Saturday by declaring that "we defeated" the terrorists in Iraq. U.S. commanders remain cautious.

Army Lt. Gen. Lloyd Austin, the No. 2 commander in Iraq, said recently that "our progress is fragile, and we continue to work to make this progress irreversible."

Such encouraging reports could benefit both presidential candidates. Republican John McCain has been a major supporter of Bush's escalation of U.S. forces in Iraq. Democratic candidate Barack Obama said he wants to withdraw all U.S. troops from Iraq in 16 months, although he said any pullout would be determined by conditions there.

Violence in Afghanistan is growing, increasing pressure to shift more troops from Iraq to there.

Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, said he wants to send more U.S. troops to Afghanistan, but he doesn't want to sacrifice gains in Iraq by shifting troops too soon.

About half of the 60,000 allied forces in Afghanistan are American.

Four of the five extra brigades sent to Iraq last year have left the country; the last unit is preparing to leave this month. The extra brigades increased U.S. troop levels to about 160,000 from 130,000. Even after five combat brigades leave, about 140,000 U.S. troops will remain in Iraq.

The average number of weekly attacks in Iraq has dropped to 200, an 80% reduction since June 2007, according to Multi-National Corps — Iraq. U.S. and Iraqi casualties have also dropped significantly.

The State Department said in a recent report that Iraq has met 15 of 18 congressional benchmarks designed to measure progress in Iraq.

The improved effectiveness of Iraq's security forces will make it easier to withdraw U.S. troops, said California Rep. Duncan Hunter, the top Republican on the House Armed Services Committee.

"I think it might surprise some people how fast we can come out of Iraq as the Iraqi army matures," Hunter said. "I think we passed the tipping point as far as the Iraqi army maturing."