North Korean leader Kim Jong Un called Tuesday for the "rapid expansion" of the country's nuclear weapons capability, citing ongoing US-South Korea military exercises that he said could "ignite a war".

"The US-ROK intensified military nexus and the muscle-flexing are the most obvious manifestation of their will to ignite a war," Kim was quoted as saying by the official Korean Central News Agency (KCNA).

"The prevailing situation requires us to make a radical and swift change in the existing military theory and practice and rapid expansion of nuclearization," he said.

The comments were made as Kim visited a naval destroyer, Choe Hyon, on Monday and received a report about the warship's weapons systems.

He expressed satisfaction that "the major tasks for making the navy high-tech and nuclear-armed" were "progressing as planned" ahead of an assessment target in October, KCNA said.

The United States and South Korea on Monday began annual joint exercises aimed at preparing for potential threats from nuclear-armed North Korea.

The eleven-day exercises include "several large-scale, live-fire training events," a US army statement said. It called the drills "a defense oriented exercise".

South Korean President Lee Jae Myung vowed on Friday to "respect" North Korea's political system and build "military trust", a day after Pyongyang said it had no interest in improving relations with Seoul.

Lee has pledged to pursue dialogue with the nuclear-armed North without preconditions since his election in June, a reversal from his hawkish predecessor.

Lee's speech came a day after North Korean leader Kim's sister, Kim Yo Jong, said the North has "no will to improve relations" with the South.

She also denied reports that North Korea was removing propaganda loudspeakers near the countries' border.

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