North Korea slammed the U.S. on Saturday after one of its aircraft carriers arrived at a South Korean port, calling the deployment a "reckless" act of "grave unpardonable provocation" intended to block the improvement of North and South Korean relations.

The nuclear-powered aircraft carrier USS George Washington arrived at the port of Busan, South Korea on Friday ahead of the annual U.S-South Korea military exercises starting next Wednesday.

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"The U.S. should properly understand that the more persistently it resorts to reckless nuclear blackmail and threat, the further the DPRK (North Korea) will bolster up its cutting edge nuclear force for self-defense," said an unnamed spokesman of the North Korean National Defense Commission's Policy Department.

He said the joint exercise was an "open challenge" to the North after it had proposed that the South suspend all hostile military activities and forgo the joint exercises with the U.S. last week.

The NDC spokesman accused Washington of unnecessary interference, implying that the U.S. did not want relations in the Korean Peninsula to improve.

"Whenever there was a sign of improving the North-South relations and détente on the peninsula, the U.S. resorted to sinister interference and obstructions," he said.

Pyongyang has made no secret of its condemnation of the South's military drills with the U.S. as preparation for an invasion.

North Korea expressed its objection to the two-month long U.S.-South Korea drills in April by firing a number of short-range missiles. It warned the U.S. that it would continue to demonstrate its power if Washington persisted in destabilizing the peninsula.

Meanwhile, Seoul dismissed Pyongyang's peace proposal as "nonsensical" and pointed out that the North's nuclear weapons program was hurting regional peace.

On Monday, Pyongyang again issued a second proposal urging the South to cease hostilities and demanded it stop annual drills with the U.S.

The U.S.-South Korea naval exercises will be held on July 16-21 after which Japan's naval forces will join the drills for two days in the waters near Jeju Island, according to Yonhap news.

TagsKorean peninsula, north-south korea relations, NoKor relations, joint military exercises, North Korea, USS George Washington