North Korean leader Kim Jong Un has said an elevated U.S. security partnership with South Korea and Japan poses a grave threat to his country and vowed to further bolster his nuclear weapons program.

North Korea on Tuesday denounced the recent arrival of a U.S. Navy attack submarine in Busan, warning that it would respond with "corresponding strength" against what it views as U.S. military intimidation.

The South Korean news agency Yonhap reported that the USS Alexandria, a US nuclear-powered submarine, had arrived at the naval base in Pusan in South Korea on Monday.

Leader Kim Jong Un pledged to pursue "sustained countermeasures," citing security ties between the U.S. and its regional allies.

North Korean leader Kim Jong Un has vowed to bolster his nuclear weapons program in response to the U.S.’s enhanced security partnership with South Korea and Japan. Last year, the U.S. expanded bilateral military exercises with South Korean and trilateral training with South Korea and Japan.

North Korea on Tuesday denounced the recent arrival of a U.S. nuclear-powered submarine in South Korea, accusing Washington of ignoring Pyongyang's security concerns and warning that it is ready to use any means to defend its national security.

North Korean leader Kim Jong Un has pledged to further develop the country's nuclear arsenal, citing increasing security threats from the United States, South Korea, and Japan. In a speech marking the 77th anniversary of the Korean People's Army,