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.

The North has been opposed to the U.S. carrier's port call, viewing it as nuclear blackmail and a military threat against it.

North Korea has also frequently accused South Korea and the U.S. of holding joint military exercises as a rehearsal for invasion of the North, a charge denied by Seoul and Washington. The two allies say their annual exercises are defensive in nature.

"The U.S. should properly understand that the more persistently it resorts to reckless nuclear blackmail and threat, the further the DPRK will bolster up its cutting edge nuclear force for self-defense always ready to go into action," the North's powerful National Defense Commission said in a statement in English, using the acronym of the North's official name.

The statement, carried by the country's official Korean Central News Agency, claimed that the planned military maneuvers are a challenge to North Korea's sincere efforts "to defuse the tension on the peninsula and create peaceful atmosphere." (Yonhap)