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The United States moved on Sunday to reassure Tokyo over its mounting security concerns, saying it would send more missile defense ships to Japan following North Korean launches and use a high level trip to warn China against abusing its "great power."

Japan has watched with alarm in recent weeks as North Korea carried out a series of missile launches, including firing two medium-range missiles capable of hitting the U.S. ally.

Tokyo has also voiced growing anxiety over China's military buildup and increasingly assertive behavior in a territorial dispute over East China Sea islands.

U.S. Defense Secretary Chuck Hagel announced that two Navy destroyers equipped with missile defense systems would be deployed to Japan by 2017. It was a response, he said, to provocations from the North, which has also threatened to carry out a "new form" of nuclear test.

The announcement followed other steps taken by the Pentagon to bolster its military posture in Japan, including an October decision to position a second X-band missile defense radar there. That radar is expected to be operational this year.

"These steps will greatly enhance our ability to defend both Japan and the U.S. homeland from North Korean ballistic missile threats," Hagel told reporters at Japan's defense ministry.

Narushige Michishita, associate professor and security expert at the National Graduate Institute for Policy Studies, said the moves were "part of the U.S. attempt to bolster reassurances vis-à-vis Japan."

It also fits within the context of broader American efforts to bolster its military presence in the region, part of a strategic "rebalance" or "pivot" toward Asia that President Barack Obama will emphasize during his trip this month to Japan, South Korea, Malaysia and the Philippines.

"GREAT POWER"

As Washington pivots, China has been ramping up military spending, building new submarines, surface ships and anti-ship ballistic missiles and testing emerging technology aimed at destroying missiles in mid-air -- technologies the Pentagon says appear designed to counter U.S. military capabilities.

China is also becoming more assertive in territorial disputes, including last year declaring an air defense identification zone covering disputed, Japanese-administered islands in the East China Sea.

Hagel, who leaves for Beijing on Monday, called China a great power, but used unusually strong language about how nations should wield such power, saying they must not resort to coercion or intimidation. That, he warned, could trigger conflict.

"Great powers have great responsibilities. And China is a great power," Hagel said, adding he wanted to talk with China about its use of military power and encourage transparency.

In remarks almost certainly meant to reassure Japan, a treaty ally that the United States has pledged to defend, Hagel pointed to the example of Russia's annexation of Crimea as the kind of action that would not be tolerated.

"You cannot go around the world and redefine boundaries and violate territorial integrity and the sovereignty of nations by force, coercion or intimidation, whether it's in small islands in the Pacific, or in large nations in Europe," he said.

Japan has drawn parallels between Russia's actions in Crimea and what it sees as China's challenge to the status quo in the East China Sea.

Hagel hosted talks last week with Southeast Asian defense ministers in Hawaii, where he also warned of growing U.S. concern about territorial disputes in the South China Sea.

The U.S. State Department has accused China's coastguard of harassing Philippine vessels and called its attempt a week ago to block a Philippine resupply mission to the Second Thomas Shoal, a disputed atoll, provocative and destabilizing.

"Something else ... that I will be talking with the Chinese about is respect for their neighbors. Coercion, intimidation is a very deadly thing that leads only to conflict," he said.

"All nations, all people deserve respect."

(Additional reporting by Linda Sieg; Editing by Michael Urquhart and Ron Popeski)