* Hagel defends U.S. policy on Russia, stresses commitment to Japan
* Says China should harbour no doubts about U.S. pledges to allies
* Visit takes place after Japanese unveils plan to overhaul ban on arms exports (Recasts with Hagel's arrival in Japan)
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.
But he rejected any suggestion of American weakness as he renewed U.S. commitments to Japan, which is locked in a dispute with China over islands in the East China Sea.
"I don't think there is any indication or any evidence that we're doing anything but strengthening our commitment to the security of Japan," Hagel said, pointing to examples like U.S. plans to put another X-band U.S. missile-defense radar in Japan.
This month, the United States will also begin rotational deployments in Japan of its Global Hawk reconnaissance drones.
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 such reassurances can on their own allay worries in Japan that Washington might one day be unable or unwilling to militarily defend the country, despite President Barack 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 South China 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.
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."
On Sunday, Hagel will hold talks with Japanese Foreign Minister Fumio Kishida and Defense Minister Itsunori Onodera.