By Phil Stewart
TOKYO (Reuters) - U.S. Defense Secretary Chuck Hagel moved on Saturday to reassure Japan of America's commitment to its security, as Russia's annexation of Crimea raises eyebrows in a region facing its own territorial disputes with an increasingly assertive China.
The United States and its allies have made clear they have no military plans to defend Ukraine, which is not a NATO member, instead moving to isolate Russia diplomatically and impose limited sanctions.
Critics say such moves are too weak to return Crimea to Ukrainian control and do little to deter further aggression.
Hagel defended the U.S. strategy to punish Russia and told reporters ahead of two days of talks with Japanese leaders that it was natural that "allies are going to look at each other to be assured", given the crisis in Ukraine.
"It's a pretty predictable, I think, reaction not just of nations of this area, of this region, but all over the world. It has to concern nations," he said.
But Hagel rejected any suggestion of weakness as he renewed U.S. commitments to Japan, which is locked in a dispute with China over islands in the East China Sea.
Washington takes no position on the sovereignty of the islands, called the Senkaku by Japan and the Diaoyu by China, but recognizes that Japan administers them and says they fall under the U.S.-Japan Security Treaty, which obligates America to come to Japan's defense.
Addressing U.S. and Japanese forces at Yokota Air Base, Hagel said one of the goals of his trip to the region was to assure allies of America's commitment to "our treaty obligations."
"We're serious about that," he said.
Daniel Russel, President Barack Obama's diplomatic point man for East Asia, said on Thursday the prospect of economic retaliation should discourage Beijing from using force to pursue territorial claims in Asia, in the way Russia has in Crimea.
He stressed that China also should not doubt the U.S. commitment to defend its Asian allies.
It is unclear if U.S. reassurances can on their own allay worries in Japan that Washington might one day be unable or unwilling to militarily defend the country, despite Obama's strategic "pivot" toward the Asia-Pacific region.
Obama is expected to visit Japan later this month.
BEEFING UP THE ARMED FORCES
Such fears have added momentum to Japanese Prime Minister Shinzo Abe's drive to beef up Japan's forces while loosening constitutional limits on military actions overseas.
His government this week unveiled an overhaul of a decades-old ban on weapons exports.
In an interview published before his arrival, Hagel said he welcomed the possibility of Japan giving its military a greater role by allowing it to come to the aid of allies under attack.
"We welcome Japan's efforts to play a more proactive role in the alliance, including by re-examining the interpretation of its constitution relating to the right of collective self-defense," Hagel said in a written response to the Nikkei, Japan's main financial newspaper.
Hagel, who travels next to China after his weekend visit to Japan, just wrapped up three days of talks with Southeast Asian defense ministers in Hawaii, where he warned of growing U.S. concern about territorial disputes in the South China Sea.
China claims about 90 percent of the sea, displaying its reach on official maps with a so-called nine-dash line that stretches deep into the maritime heart of Southeast Asia. The Philippines, Vietnam, Malaysia, Brunei and Taiwan also have claims to parts of the potentially energy-rich waters.
"We have differences (with China). And the only way to deal with differences is (to be) straight up honest, talk about it and deal with it," Hagel told U.S. and Japanese forces.
Russia's annexation of Crimea came up in discussions at the Hawaii talks, one senior U.S. defense official acknowledged. But the official played down the extent of discussions, saying there "wasn't a lot of hand wringing."
Ukraine gave up its Soviet-era nuclear arsenal under the 1994 Budapest Memorandum, which it signed together with Britain, United States and Russia. It provided guarantees of Ukraine's sovereignty in exchange for a commitment, since fulfilled, to give up the country's nuclear weapons.
(Additional reporting by Tim Kelly; Editing by Nick Macfie)