(Updates throughout with fresh quotes and details)

By David Morgan

WASHINGTON, Aug 27 (Reuters) - The top U.S. Marine officer said on Wednesday he could reduce his 25,000-strong force in the former al Qaeda stronghold of Iraq's Anbar province to reinforce military operations against a growing Taliban threat in Afghanistan.

Marine Corps Commandant Gen. James Conway told reporters the once-restive province west of Baghdad could be turned over to Iraqi security control within days, thanks to the sharp decline in violence that occurred when Sunni tribal leaders switched allegiance from al Qaeda to the U.S. military.

The Marine Corps Times said on its Web site that Anbar security would revert to Iraq next week. Marine officials declined to confirm a specific date due to security concerns.

"The requirement right now in Iraq is much more about nation-building than it is fighting," Conway said at a Pentagon briefing.

"It's our view that if there's a stiffer fight going on someplace else ... then that's where we need to be."

U.S. and NATO forces in Afghanistan face an intensifying insurgency marked by escalating attacks and military casualty rates that have helped make Afghanistan a deadlier place than Iraq for U.S. troops in recent months.

Conway said air power would continue to play a primary role despite the risk of civilian casualties that have angered Afghans and made U.S. and NATO forces more unpopular.

He said it was unclear how many civilians died in an Aug. 21 air strike in western Afghanistan, despite a U.N. finding of evidence that about 90 were killed, most of them children. But the general accused the Taliban of operating among civilians to reap a propaganda advantage from military attacks.

"This is a dirty game being played," Conway said.

"Air power is the premier asymmetric advantage that we hold over ... the Taliban. They have no like capability," he said. "We'll continue to drop bombs. We will also continue at every effort to preserve civilian lives who unfortunately are a part of the battlefield."

A U.S. military official declined to say which branch of the U.S. armed forces carried out the Aug. 21 attack.

The United States has 33,000 troops in Afghanistan, including 3,400 Marines who are due to leave the country by the end of November.

Two Marine regimental combat teams deployed in Iraq are part of a U.S. force of about 146,000 and were sent to Anbar at the height of a Sunni insurgency centered in the province.

'LESSER NUMBERS'

Conway said the size of any Marine deployment to Afghanistan would be smaller than the one now on duty in Iraq.

He declined to recommend a specific troop number but said the corps ultimately would like to have 15,000 troops deployed worldwide. There are currently 34,000 Marines on worldwide deployment, only 5,600 of whom are deployed neither to Iraq nor Afghanistan.

U.S. defense officials have long recognized the need to redeploy troops from Iraq to Afghanistan but no final decision has been made.

Defense Secretary Robert Gates and other top Pentagon officials are considering ways to increase the number of U.S. combat brigades in Afghanistan to confront the Taliban.

So far, the Pentagon has taken only small steps by ordering one-month tour extensions for Marines and deploying less than 200 additional support troops.

Conway suggested a drawdown of Marines in Iraq could allow for the replacement of about 1,200 troops from the 2nd Battalion, 7th Marine Regiment that are in the country until Nov. 30 to train Afghan security forces.

But he said it was unlikely that fresh Marine forces would be deployed to replace the 2,200 Marines fighting Taliban insurgents in southern Afghanistan. (Editing by David Alexander and Peter Cooney)

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