By David Alexander

WASHINGTON (Reuters) - North Korea is on a quest to develop nuclear-armed ballistic missiles that can strike the United States, and the best way to counter Pyongyang is with combat power and strong alliances, the commander of U.S. forces in the Pacific said on Tuesday.

Admiral Harry Harris, the head of U.S. Pacific Command, told a South Korea support group in Washington that no threat in the region was "more dangerous than North Korea" and its leader Kim Jong-un.

"He recently threatened pre-emptive nuclear strikes against the United States and other countries in the region," Harris said. "Folks, make no mistake about it. North Korea is on a quest for nuclear weapons, the means to miniaturize them and the ways to deliver them intercontinentally."

Harris said the threat from Pyongyang was one reason he has been working to boost trilateral cooperation among South Korea, Japan and the United States. Military cooperation among the three "is good, and I'd like to get it better," he said.

Harris said the recent settlement of the Second World War "comfort women" issue between Japan and South Korea would help enable better cooperation.

Japanese Prime Minister Shinzo Abe's new defense guidelines allowing Tokyo to play a more robust role in security affairs also will help, he added.

"Working trilaterally, I believe we can bolster our collective defense against North Korean provocations," Harris said.

He said U.S., Japanese and South Korean diplomats plan to expand cooperation due to North Korea's aggressive nuclear and missile testing program this year. They also decided to build on a trilateral information-sharing arrangement aimed at countering Pyongyang's missile threats.

Under the umbrella of that deal, they plan to conduct a missile defense exercise on the sidelines of the Rim of the Pacific naval maneuvers this summer, Pentagon Press Secretary Peter Cook said this week.

Harris said the exercise would improve their ability to "operate together in a ballistic missile defense environment."

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U.S. and South Korean officials are currently discussing whether to deploy the U.S. THAAD missile defense system on the peninsula. Harris said that decision would be made jointly by the alliance.

Regarding China's island-building in the South China Sea, Harris said Washington had more areas of constructive collaboration with Beijing than it had disputes. But he noted China sometimes engaged in "provocative and aggressive behavior."

"We should cooperate with China wherever we can, but we have to confront China where we must," Harris said. "Some of their activities in the South China Sea falls into that category."

(Reporting by David Alexander; Editing by Jonathan Oatis)