Obama aides see need for more troops in Afghanistan...

* White House wariness reflects waning public support

* Obama needs to prepare U.S. public for troop increase

* Commanding general calls for fresh strategy

By Adam Entous and Arshad Mohammed

WASHINGTON, Aug 31 (Reuters) - Many of President Barack Obama's top advisers on Afghanistan agree with military commanders that more troops are needed to reverse Taliban gains in the country's east and south, U.S. officials said on Monday.

But there is wariness within the White House to another large-scale increase at a time when public support for the eight-year-old war against a resurgent Taliban is eroding, the officials said, speaking on condition of anonymity.

Military commanders and administration and congressional leaders have held preliminary discussions about future troop options, including sending a second 5,000-member Marine Regimental Combat Team to southern Afghanistan, a Taliban stronghold, participants said. This would boost the number of Marines in the country to 15,000-18,000 from just over 10,000.

The debate is expected to intensify after Monday's long-awaited assessment of the war by U.S. Army Gen. Stanley McChrystal, commander of U.S. and NATO forces in Afghanistan.

McChrystal called for the United States and its allies to change strategy, laying the ground for a likely request for more troops later, officials said. [nISL129837]

McChrystal has about 103,000 troops under his command, including 63,000 Americans, half of whom arrived this year as part of an escalation strategy started by former President George W. Bush and ramped up under Obama.

The force is set to rise to 110,000, including 68,000 Americans, by year's end, stretching the U.S. military to its limits, military officials said.

U.S. officials said further troop increases would hinge in part on the pace at which combat brigades could be pulled out of Iraq and redeployed to Afghanistan.

Another key factor, the officials said, was whether Obama would make a concerted effort to overcome growing public opposition to the war, fueled by record U.S. combat deaths.

Pressure from within the president's Democratic party for a withdrawal timeline is expected to increase in the run-up to next year's mid-term U.S. congressional elections.

"There is great awareness over at the White House ... that support in the public is really declining," one official said.

Another U.S. official said Obama had not yet prepared the American people for what many top advisers see as an inevitable need to increase the number of troops in Afghanistan.

"The question is not only can you rotate a sufficient number out of Iraq," he said "What the administration has to do is politically make sure that they prepare the ground for it."

"Half-measures are not going to work," the official added. "They haven't worked in the past."

HARD-SELL

It is unclear how much room Obama has to maneuver. With his popularity dented by a raucous debate over healthcare reform and the electorate still shaken by the recession, Obama may also be loath to push an unpopular policy right now.

U.S. Defense Secretary Robert Gates said on Monday McChrystal should be "forthright" about spelling out what he needs in terms of troops and equipment, but he also made clear that another major troop increase would face hurdles.

"I have expressed some concerns in the past about the size of the American footprint, the size of the foreign military footprint, in Afghanistan, and clearly I want to address those issues," Gates said during a visit to Fort Worth, Texas.

"And we will have to look at the availability of forces, we'll have to look at cost. There are a lot of different things that we'll have to look at once we get his recommendations, before we make any recommendations to the president."

Obama's advisers on Afghanistan are particularly sympathetic to pleas for more resources in eastern Afghanistan near the border with Pakistan, where veteran Taliban leader Jalaluddin Haqqani is now seen as the main threat.

A rapid deterioration in the war in the east has taken U.S. officials by surprise. In April, a senior commander said NATO forces were close to achieving "irreversible momentum" there.

Anthony Cordesman, an expert with the Center for Strategic and International Studies in Washington, said "strong elements" in the White House, State Department and other agencies were pressing Obama to avoid sending more troops and money.

"If these elements succeed, President Obama will be as much a failed wartime president as George W. Bush," he wrote in Monday's Washington Post, saying such an approach would condemn the United States to "certain defeat