By Matt Spetalnick and Manuel Mogato

WASHINGTON/MANILA (Reuters) - When a Philippine government ship evaded a Chinese blockade in disputed waters of the South China Sea last month, a U.S. Navy plane swooped in to witness the dramatic encounter.

The flyover was a vivid illustration of the expanding significance of one of Asia's most strategic regions and underscored a message that senior U.S. officials say President Barack Obama will make in Asia next week: The "pivot" of U.S. military and diplomatic assets toward the Asia-Pacific region is real.

Washington's Asian allies, however, appear unconvinced.

During Obama's four-nation tour of Asia that begins on April 23, his toughest challenge will be to reassure skeptical leaders that the United States intends to be more than just a casual observer and instead is genuinely committed to countering an increasingly assertive China in the region.

Russia's annexation of Ukraine's Crimea peninsula - and perceptions of limited U.S. options to get Moscow to back down - has heightened unease in Japan, the Philippines and elsewhere about whether Beijing might feel emboldened to use force to pursue its territorial claims in the East and South China Seas.

There is also suspicion among some Asian allies that if they come under threat from China, the United States - despite treaty obligations to come to their aid - might craft a response aimed more at controlling damage to its own vital relationship with China, the world's second-biggest economic power.

For Obama, the tricky part of the trip, which will include stops in Japan, South Korea, Malaysia and the Philippines, will be deciding how to set limits on China in a way that soothes U.S. allies in Asia but avoids stoking tensions with Beijing.

"Obama's upcoming visit will be the most critical test of this administration's Asia policy," said Richard Jacobson, a Manila-based analyst with TD International, a business risk and strategic consulting firm.

Story continues

U.S. officials say the Obama administration's long-promised "rebalancing" of America's economic, diplomatic and security policy toward Asia is on track, largely unaffected by the attention demanded by the crisis in Ukraine or persistent troubles in the Middle East.

The Asia "pivot" - as the White House initially dubbed it - represented a strategy to refocus on the region's dynamic economies as the United States disentangled itself from costly wars in Iraq and Afghanistan.

But doubts about Washington's commitment to Asia are simmering in some allied capitals.

"It was a welcome policy change, but will they do it?" Yukio Okamoto, a former Japanese government adviser on foreign affairs said of the strategic shift toward Asia that Obama announced in 2011. "We do not see any actual sign" of its implementation.

When Obama announced the eastward shift, the most dramatic symbol of the new policy was the planned deployment of 2,500 U.S. Marines in northern Australia, where they would be primed to respond to regional conflicts. It took until this month to build up forces to 1,150 Marines based in Darwin, and the full contingent is not due to be in place until 2017.

"The U.S. pivot towards Asia has had very few tangible, concrete outcomes so far," said Adam Lockyer, a foreign policy and defense analyst at the University of New South Wales.

A SIGN OF ANXIETY

Obama will try to put those concerns to rest while in Manila, where Philippine officials say he is expected to sign a security pact that will allow for increased use of Philippine bases by U.S. ships, aircraft and troops.

Manila's acceptance of a beefed-up U.S. military presence, a politically sensitive issue in the independent-minded archipelago nation, would reveal the scale of Philippine anxiety over China.

The Philippine Senate voted to evict the U.S. military from their bases in 1991, ending 94 years of American military presence in the Philippines, and has only gradually allowed the return of U.S. forces for limited operations during the past decade.

The Philippine government is struggling to keep control of Second Thomas Shoal, where it has a military outpost on a reef surrounded by Chinese coastguard ships. The outpost itself is a huge, rusting World War Two transport vessel that the Philippine navy intentionally ran aground in 1999 to mark its claim.

Eight or so Filipino soldiers live there for three months at a time in harsh conditions on a reef that Manila says is within its 200-nautical mile exclusive economic zone. China, which claims 90 percent of the South China Sea, says the shoal is part of its territory.

Last month, a U.S. surveillance plane was spotted overhead as a Philippine vessel dodged Chinese coastguard vessels to deliver supplies and fresh troops to the outpost.

Such U.S. aircraft normally conduct their missions at higher altitudes, so the fact that its flyover was in full view of journalists monitoring the incident on the surface below suggested that the United States wanted to make its presence known. A Chinese plane and a Philippine military aircraft also flew above the area at different intervals.

The administration has promised that the United States will reposition naval forces so that 60 percent of its warships are based in Asia-Pacific by the end of the decade, up from about 50 percent now. But as the U.S. military budget contracts, that likely would represent part of a shrinking U.S. defense pie.

Obama's aides brush aside complaints about the U.S. follow-through on the pivot strategy, saying that no matter how much attention Washington devotes to friends and partners in the region, the allies will always want more from their superpower friend.

"Questions by Asia-Pacific allies about the degree of American commitment has been a constant component of our relationship for 60-plus years. It's not new," said a senior U.S. official, who asked not to be identified because he was not authorized to comment publicly. "It doesn't mean the U.S. won't do more to work with them."

MAKING UP FOR OBAMA'S NO-SHOW

Obama himself helped to fuel some of the skepticism about the United States' commitment to Asia when he abruptly canceled a long-planned trip to Asia to attend two regional summits last fall and stayed home to deal with a U.S. government shutdown.

Since then, negotiations have dragged on over a proposed U.S.-led trans-Pacific trade pact that is widely seen as the economic centerpiece of Obama's pivot strategy.

In this tense regional climate, Obama can be expected to appeal directly to Asian leaders to have faith in America's resolve to keep China in check and discourage any notion that Beijing could emulate Russia's takeover of Crimea by seizing contested islands and shoals from its neighbors.

"Among countries in Asia, there has been an increase in the level of anxiety about what lessons China may be drawing from Russia and Ukraine," the senior U.S. official said.

While sticking to a U.S. refusal to take sides in the maritime disputes, Obama will seek to reassure South Korea, Japan and the Philippines that Washington is "fully committed to our defense treaties" with them, the official said.

Obama's Japanese hosts likely will only be satisfied if the president takes a tough stand against China and in solidarity with Japan amid growing concern that Washington's defense commitment may be wavering.

Tokyo and Beijing are locked in a bitter row in the East China Sea over tiny, uninhabited isles administered by Japan, especially since China announced the creation of a controversial new air defense zone covering the area, which the Japanese call Senkaku and the Chinese call Diaoyu.

Relations between Japan and China have been further poisoned

by Japanese Prime Minister Shinzo Abe's visit to a controversial shrine for war dead seen as a symbol of Japan's past militarism - a move that drew U.S. criticism as well.

Obama, the U.S. official said, will send a message during his Asian tour to China that "it should not use intimidation or coercion" against its neighbors.

That is not likely to go down well in Beijing, where visiting U.S. Secretary of State Chuck Hagel faced harsh accusations last week from Chinese officials who claimed that Washington's regional agenda was aimed at blocking China's rise.

(Additional reporting by Greg Torode in Hong Kong, Linda Sieg in Tokyo, Matt Siegel in Sydney, David Brunnstrom and Mark Felsenthal in Washington; Editing by Jason Szep and David Lindsey)