SEOUL (Reuters) - South Korea, Japan and the United States staged long-planned joint naval exercises involving an American aircraft carrier to ensure readiness against nuclear and missile threats from North Korea, Seoul's navy said on Friday.

       North Korea has been accelerating weapons development, testing solid-fuel hypersonic missiles last week, and leader Kim Jong Un said on Wednesday now was the time to be more prepared for war than ever, citing an unstable geopolitical situation.

       "The participating forces conducted anti-submarine warfare drills to improve their responses to North Korean underwater threats including from submarines and submarine-launched ballistic missiles," the South's navy said in a statement.

       The two-day drills held in international waters between South Korea and Japan from Thursday brought together the air carrier Theodore Roosevelt and the destroyers Howard, Russell and Daniel Inouye, with warships from the Asian neighbours.

       The training, aimed to step up their combined capability to respond to North Korea's threats, follows a multi-year joint exercise plan set up after last year's three-way summit, the South Korean navy said.

       The three navies also practised maritime interdiction exercises designed to block North Korea's illegal transport of weapons of mass destruction, and search and rescue training to help ships in distress.

       In Washington, South Korean and U.S. defence officials held annual defence talks on Thursday, reaffirming plans for tabletop exercises simulating North Korea's use of a nuclear weapon in upcoming summer drills, Seoul's defence ministry added.

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       At the Camp David summit last August, leaders of the three countries agreed to stage annual, multi-domain exercises, share real-time information on North Korean missile launches and launch a hotline for crisis communications.

       (Reporting by Hyonhee Shin; Editing by Clarence Fernandez)