By Linda Sieg
TOKYO (Reuters) - U.S. President Barack Obama arrives in a tense Asian region on Wednesday, faced with the delicate task of assuring Japan and other regional allies of America's commitment to their defense without hurting Washington's vital ties with a rising China.
That difficult diplomatic balancing act was highlighted on Monday, when Japanese Prime Minister Shinzo Abe sent a ritual offering to Tokyo's Yasukuni Shrine, seen in parts of the region as a symbol of Japan's past militarism.
The move strained Tokyo's already tense ties with China and fellow U.S. ally South Korea, another stop on his four-nation tour that will also take in Malaysia and the Philippines.
Japan, for its part, has been beset by anxiety over the degree to which reality matches rhetoric in Obama's promised "pivot" of U.S. military and diplomatic assets to Asia.
Abe and Obama will be keen to send a message that the alliance - central to America's presence in Asia and the core of Tokyo's security policy - is stronger than ever when they hold their symbolic summit on Thursday.
"The fundamental message the two leaders are trying to send is solidarity in the face of China's assertive behavior, and I expect that message to be fairly clear," said a former Western diplomat. "In that sense, an element of success is very likely."
The two leaders are also likely to discuss how to deal with North Korea at a time when the region is jittery over a possible nuclear test by an unpredictable Pyongyang.
North Korea, already subject to United Nations' sanctions over its previous atomic tests, the third and most recent of which took place in early 2013, threatened last month to conduct what it call "a new form of nuclear test".
On Monday the North's KCNA news agency quoted a foreign ministry spokesman saying Obama's trip was a "reactionary and dangerous one as it is aimed to escalate confrontation and bring dark clouds of a nuclear arms race to hang over this unstable region".
U.S.-Japan relations were strained after Abe in December visited the Yasukuni Shrine, where wartime leaders convicted as war criminals by an Allied tribunal are honored along with war dead. The visit prompted a U.S. statement of "disappointment".
Abe has since sought to soothe U.S. concerns that his conservative agenda to recast wartime history with a less apologetic tone is blocking improved ties with Seoul and giving China ammunition to paint him as reviving past militarism.
Last month, Abe told parliament that he has no plans to revise a landmark 1993 apology to women, many Korean, forced to work in Japan's wartime military brothels.
And while he sent a ritual offering to Yasukuni on Monday, Abe did not join the nearly 150 lawmakers who visited in person to commemorate its spring festival.
"Abe, by declining to visit Yasukuni for the spring festival, sent the message that he has heard the U.S., that the message has been received," the ex-diplomat said. "To that degree, the situation is different from some months back."
Still, Obama - in Japan on the first full state visit by a U.S. president since Bill Clinton in 1996 - must tread carefully, both to avoid providing fodder from critics alert to the tiniest sign of cracks in the alliance, while avoiding inflaming Chinese anger.
"Obama has to assure Japan that the alliance is rock-solid but he can't do too much to alienate or provoke China, so it's a delicate situation," said Jeffrey Kingston, director of Asian studies at Temple University's Japan campus.
Abe is also anxious to ease the restrictions placed on Japan's military by its pacifist, post-war constitution, a move that would be welcomed by Washington for allowing Tokyo to shoulder a greater burden in their security alliance.
Sino-Japanese relations have chilled markedly over the past two years due, in part due to a bitter dispute over rival claims to tiny, uninhabited islands in the East China Sea, known as the Senkaku in Japan and the Diaoyu in China.
Japanese and Chinese patrol ships have been playing cat-and-mouse in the disputed waters and Japanese fighter jets scrambles against Chinese planes hit a record high in the year to March 31, stirring concerns that an unintended clash could escalate.
Washington says the Japanese-administered islands fall under a U.S.-Japan treaty that obligates it to defend Japan, but it takes no position on their sovereignty and is wary of being dragged into a Sino-Japanese military clash.
A joint statement to be issued at the summit will state that the two allies will not tolerate any attempt to change the status quo by force - a phrase that implicitly targets China - but not mention the islands by name, Japanese media reported.
The two leaders will also need to show progress towards a two-way trade pact seen as vital to a broader U.S.-led Trans-Pacific Partnership (TPP) deal.
The deal is both a pillar of Obama's Asia rebalancing and critical to Abe's growth strategy, the "Third Arrow" of his recipe to revive the Japan's moribund economy alongside hyper-easy monetary policy and increased fiscal spending.
But doubts persist over whether they can reach even the outlines of a bilateral trade agreement, given that significant gaps remain over Japan's desire to keep tariffs on politically sensitive farm products such as beef.
Failure could take the wind out of the push for a broader agreement among the 12-nation TPP group that would stretch from Asia to Latin America.
Some trade experts said that despite the hurdles, a last-minute agreement could not be ruled out. Abe and Obama will dine together on Wednesday night, a U.S. official said, and Japanese media have reported that U.S. Trade Representative Michael Froman and Japanese Economy Minister Akira Amari might join.
"I think there is still a chance that Japan and the U.S. will reach a broad agreement on TPP through a political decision between Prime Minister Abe and President Obama this week," said Kazuhito Yamashita, research director at the Canon Institute for Global Studies. "It will be one important symbol to show off a strengthened Japan-U.S. alliance to China."
(Additional reporting by Kaori Kaneko; Editing by Alex Richardson)