* Worries over U.S. commitment, Japan's less apologetic tone on war
* Increasingly assertive China in dispute with Japan over territory, war
* Economic centrepiece of Obama's Asia strategy snagged with Japan
* Japan postpones foreign minister visit to Moscow (Adds postponement of foreign minister's trip to Moscow)
By Linda Sieg and David Brunnstrom
TOKYO/WASHINGTON, April 17 (Reuters) - When U.S. President Barack Obama meets Japanese Prime Minister Shinzo Abe at a summit next week, they will be battling negative undercurrents that could undermine their message that Asia's most important security alliance is firm.
Obama, stopping in Tokyo at the beginning of a four-nation Asian trip, must counter worries in Japan that his commitment to its defence in the face of an increasingly assertive China is weak.
Abe will be trying to soothe U.S. concerns that his conservative push to recast Japan's war record with a less apologetic tone is overshadowing his pragmatic policies on the economy and security.
The two leaders will also need to demonstrate at least the prospect of progress on the economic centrepiece of what the Obama administration calls its strategic "rebalance" to Asia - a Trans-Pacific Partnership trade pact that would bring together Japan, the United States and 10 other economies.
Strains could also emerge over the crisis in the Ukraine, expected to be on the agenda when the two leaders talk.
Japanese and U.S. officials say the alliance is rock-solid and the atmospherics will be just fine at what will be the first state visit to Tokyo by a U.S. president since Bill Clinton in 1996.
But in a sign of how imagery matters, Tokyo sweated for months for Washington to affirm Japan's special ally status by agreeing to make Obama's visit a state occasion with accompanying ceremony, including dinner with the emperor, rather than a less elaborate business trip.
Obama will spend two nights in Tokyo, but almost all his events will take place on April 24, according to a provisional schedule released by Japan's foreign ministry.
"A presidential visit is at least half symbolic," said Toshio Nakayama, a professor at Keio University in Tokyo.
"We always pay attention, but we will be paying more attention to the dissonance between the two leaders."
The United States has welcomed efforts by Abe to revive Japan's economy after a two-decade slump and to shoulder more of the burden for the decades-old alliance, the core of Japan's security policies and central to the U.S. presence in Asia.
HISTORY LESSONS, UKRAINE STRAIN
But ties were strained after Abe paid his respects last December at Tokyo's Yasukuni Shrine, seen by critics as a symbol of Japan's past militarism because leaders convicted by an Allied tribunal as war criminals are honoured there along with those who died on the battlefield.
Washington had signalled it wanted Abe to stay away and then expressed "disappointment" over his visit, fearing it would further fray ties with China and South Korea, where Japan's past aggression has left a bitter legacy.
Japan is also embroiled in territorial feuds with its two neighbours, most notably a tense row over tiny East China Sea islets controlled by Japan but also claimed by China.
China in particular has used the Yasukuni visit to portray Abe as a right-wing nationalist who could lead Japan down the same path it travelled to World War Two.
A U.S. official dismissed that assessment, but also told Reuters: "We think it's important that Japan address historical issues in a way that does not increase tensions."
Some lawmakers close to Abe have resented the U.S. public chastisement over Yasukuni, which they said tilted towards Beijing and Seoul at Tokyo's expense.
Abe's government has been at pains to distance itself from such comments and others casting doubt on the prime minister's commitment to past apologies for Japan's wartime deeds. But some knowledgeable sources say the episode still rankles in Tokyo.
"The U.S. delivered a moral judgment, saying it's disappointed," said a former Japanese diplomat. "That didn't happen with previous administrations."
The more recent crisis in Ukraine could also expose divergent geopolitical interests.
Despite strong Group of Seven unity over imposing sanctions on Russia for its annexation of Crimea in southern Ukraine last month, it did not go unnoticed in the White House that Japan's sanctions were less robust than U.S. or European steps.
The topic could fan tension if Obama presses for more measures covering the oil and gas trade, given Abe's push for warmer ties with Moscow as Japan seeks to diversify its energy imports after the 2011 Fukushima nuclear disaster.
"I do not think we are out of sync now, but we could be if it is necessary to increase sanctions," said Michael Green, a member of the White House National Security Council in the George W. Bush administration and now an Asia expert at the Center for Strategic and International Studies in Washington.
JAPAN AS "RED STATE"
On Thursday, as the Ukraine crisis deepened, Japan said it was postponing a visit to Moscow by Foreign Minister Fumio Kishida that had been planned for this month, citing scheduling reasons.
Some in Japan, meanwhile, worry Obama's seeming inability to rein in Russian President Vladimir Putin is sending a message of weakness to China, which might then be emboldened to put military muscle behind its claims to the disputed East China Sea islands, known as the Senkaku in Japan and the Diaoyu in China.
The United States says the islands fall under a U.S.-Japan security pact that obligates it to defend Japan, but is wary of being drawn into any military clash between China and Japan.
One view on Crimea in Japan is that "the lack of a strong U.S. response demonstrates that the U.S. global role as a world policeman is dwindling", said Narushige Michishita, a professor at Japan's National Graduate Institute for Policy Studies.
Compounding Japanese frustration is a sense that Washington does not fully appreciate Abe's efforts to shed the constraints of Japan's post-war pacifist constitution so that it can do more in its own defence and that of its allies.
A widely held Japanese perception that ties would be warmer with a Republican in the White House - a view some close to Abe have taken no pains to hide - has also added to the discontent.
"The idea has caught on that the Democrats are pro-China and the Republicans are pro-Japan and if only the Republicans were in the White House, everything would be hunky-dory," said Columbia University professor Gerry Curtis.
Tensions born of Abe's effort to be more than a junior partner in the alliance also appear to be at work.
"I think the United States places a very high level of importance on the alliance, but thinks the ally should take orders from the boss - and they are puzzled why Japan is not taking orders," one Japanese government official told Reuters.
Conscious that China will be studying the choreography in minute detail, summit planners will do everything possible to ensure no strains are on display in Tokyo.
"Both Prime Minister Abe and President Obama are very business-like people," one Abe administration source told Reuters, dismissing talk that the two leaders do not have good personal chemistry.
"Every time they meet they engage very smoothly and are building a firm relationship of trust." (Additional reporting by Fred Katayama in New York and Jeff Mason, Matt Spetalnick and Krista Hughes in Washington; Editing by Dean Yates)