TOKYO (Reuters) - In public, Japanese Prime Minister Shinzo Abe’s government lists a more assertive China and a volatile North Korea as its top security concerns.

Soldiers from the U.S. Marines and Japan's Ground Self-Defense Force attend their joint exercises that are intended to recover an island in San Clemente Island, California, in this photo taken and provided by Kyodo February 9, 2013. REUTERS/Kyodo/Files

Behind the scenes, though, another concern is growing: that the United States may one day be unable or unwilling to defend Japan, interviews with Abe advisers, politicians and security experts show. The worries are adding momentum to Abe’s drive to beef up Japan’s air and naval forces while loosening constitutional limits on action its military can take abroad.

Japanese angst over the country’s security alliance with Washington follows years of double-digit defense spending increases by Tokyo’s arch rival in Asia, China. Unpredictable North Korea, whose missiles can hit Japan, has meanwhile pushed ahead with nuclear and missile programs despite international sanctions.

“If you are a strategic thinker or alliance planner you have to be ready for the worst case scenario,” a former Japanese diplomat close to Abe told Reuters, citing concerns about a decline in U.S. military capability and readiness.

“We should discuss roles and missions, including the kinds of weapons we have or don’t have,” added the ex-diplomat, who requested anonymity because he doesn’t hold an official position.

Conservatives like Abe also long for greater autonomy from Washington - although no one suggests that Japan, host to nearly 50,000 U.S. troops, will go it alone.

“The U.S.-Japan alliance is the most important alliance, and that will not change,” said Yosuke Isozaki, a national security adviser to Abe. “But Japan will become more of an adult, a normal country.”

Japan has even begun studying whether to boost its limited ability to make a pre-emptive strike on enemy bases, although such a costly and controversial step seems unlikely soon.

Besides strengthening its own capabilities, Japan is seeking closer security ties with Southeast Asia, India, Australia and even Russia as a hedge against any U.S. decline.

Washington routinely seeks to reassure Tokyo that the six-decade-old alliance is firm.

“The U.S.-Japan alliance is the cornerstone of regional security and prosperity,” said a senior U.S. defense department official in Washington. “The U.S. government remains committed to enhancing the U.S.-Japan alliance and upholding our obligations of the security treaty.”

Despite such assurances and President Barack Obama’s decision to strategically rebalance U.S. forces to the Asia-Pacific, Tokyo still worries whether Washington can maintain the will and wherewithal to defend Japan.

That is partly due to the perception in Japan that U.S. power is declining longer term as China’s clout grows and the growing importance of Sino-U.S. economic ties.

Japanese diplomats hope an expected Obama visit in April will ease such concerns and give the president a chance to show China, embroiled in a bitter territorial row with Japan over islands in the East China Sea, whose side Washington is on.

The Obama visit “is a vital opportunity for the United States to express its vision of what role the United States is going to play”, Japanese ambassador to Washington Kenichiro Sasae told a seminar recently. “We also want to see the United States make clear who are the friends and allies and troublemakers and potential problem-makers.”

CONSERVATIVE DREAM

The United States takes no position on the sovereignty of the islands in the East China Sea but recognizes they are administered by Japan and are covered by the security treaty, which obligates Washington to defend its ally.

Worries about the alliance also provide a justification for a conservative agenda that Abe inherited from his grandfather Nobusuke Kishi, which seeks a more equal U.S. partnership.

Kishi, a prewar cabinet minister who was jailed but never tried as a war-crimes suspect, became premier in 1957 but was forced to resign three years later after ramming the U.S.-Japan security treaty through parliament.

Abe wants to fulfill his grandfather’s goal of revising the U.S.-drafted constitution, which conservatives say restricts Japan’s ability to defend itself. Shorter-term, he hopes to loosen the constitution’s constraints by re-interpreting it.

“The fact that Obama is cutting defense, is having trouble governing at home and seems distracted by the Middle East is validating Abe’s agenda,” said Michael Green, Japan chair at Washington’s Center for Strategic and International Studies.

“Abe is pro-American but he also advocates Japan having more autonomy.”

Japan-U.S. relations hit a bump when Abe visited Tokyo’s Yasukuni Shrine in December, further straining ties with China and South Korea, which see Yasukuni as a symbol of Tokyo’s past militarism because it honors wartime leaders along with millions of war dead.

The visit prompted a rare statement of “disappointment” from the United States, which worries the feud over the islands could drag it into a military clash between Beijing and Tokyo.

Some Japanese politicians were clearly displeased.

“America said it was ‘disappointed’ but rather than being so sensitive about China’s feelings, they should be sensitive about the feelings of their ally, Japan,” said Abe aide and ruling Liberal Democratic Party (LDP) lawmaker Seiichi Eto.

“We were the ones who were ‘disappointed’.”

AIDING ALLIES, PRE-EMPTIVE STRIKE?

The two allies have begun revising guidelines on defense cooperation last updated in 1997, aiming to complete the revamp by the end of this year. Washington has long encouraged Tokyo to assume a greater share of the bilateral security burden, and Japanese policymakers now hope that by doing so, they can help anchor the United States to their side.

How far Japan can boost its role, however, depends partly on whether Abe can end Japan’s self-imposed ban on collective self-defense, or militarily aiding an ally under attack.

Changing a decades-old interpretation of the constitution to allow Japan to exercise its right of collective self-defense “would deepen the U.S.-Japan alliance, which until now, has been too one-sided”, said security adviser Isozaki.

Abe favors the change, which a panel of advisers is expected to propose as soon as April.

Some advisers say Japan’s military should not only be able to aid Washington but also countries sharing strategic interests with Tokyo. Abe’s more dovish coalition partner, the New Komeito party, is wary of any change.

Alliance anxiety has also stoked calls for Japan to strengthen its limited ability to make pre-emptive strikes.

Japan has relied entirely on the United States for deterrence even as threats from China and North Korea have grown, said former defense minister Gen Nakatani who now serves as an LDP deputy secretary-general.

“If you think about what would happen if the United States withdrew, we must consider (acquiring) the capability to respond, because we cannot just sit idly and await death.”

But with collective self-defense already on his agenda, Abe appears unlikely to devote political capital now to a cause that would be viewed as provocative by China and might not be fully welcomed by Washington.

U.S. officials have not made clear if they want Japan to acquire greater offensive capability.

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Former defense official Kyouji Yanagisawa, who worked for Abe during his 2006-2007 first term as prime minister, said doing so would “fundamentally change the nature of the U.S.-Japan alliance”, often described as Japan holding the defensive “shield” while America supplies the offensive “sword”.

Acquiring such capabilities - including cruise missiles, attack drones, geostationary satellites and even special operation forces able to penetrate enemy territory - would also be costly. Abe plans to increase defense spending by 2.6 percent over the next five years, but the spending is constrained by Japan’s massive public debt.

“I think the government is thinking about this,” said former foreign minister Seiji Maehara, a hawkish opposition lawmaker who backs much of Abe’s security agenda. “But quite a substantial package would be needed and I don’t think there is the money for that.”