N. Korea vows to boost nuclear forces against U.S. threat

North Korea threatened Saturday to strengthen its nuclear forces as a U.S. nuclear-powered aircraft carrier prepares to take part in joint military exercises with South Korea and Japan.

The 97,000-ton supercarrier USS George Washington arrived at South Korea's southeastern port city of Busan on Friday to participate in naval drills with the South Korean Navy in the country's southwest sea from July 16-21.

The carrier is also set to participate in a search and rescue exercise with South Korean and Japanese maritime forces in waters south of the southern island of Jeju for two days starting on July 21.

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