North Korean leader Kim Jong-un called for bolstering naval forces as he visited the country's navy command, while denouncing South Korea, the United States and Japan for stepping up their trilateral military cooperation, according to the North's state media Tuesday.

Kim's visit, which took place on Navy Day in North Korea, came days after he visited a navy unit where he inspected a cruise missile test aboard a warship.

"To achieve the successes in rapidly developing the naval force has become a very urgent issue in view of the enemies' recent aggressive attempts and character of military actions," Kim was quoted as saying by the North's official Korean Central News Agency in an English-language report.

Claiming that the strengthening security cooperation among Seoul, Tokyo and Washington has made waters off the Korean Peninsula the "most unstable waters with the danger of a nuclear war," Kim called for the navy to maintain war readiness, it said.

Kim also lashed out at the leaders of South Korea, the U.S. and Japan, calling them "gang bosses," in an apparent criticism of their first stand-alone summit at the Camp David presidential retreat in Maryland earlier this month.

The three leaders reached a series of agreements during the summit, including a commitment to consult one another expeditiously in the event of common threats, hold annual joint military exercises and pursue enhanced ballistic missile defense cooperation to counter North Korea's nuclear and missile threats.

"The prevailing situation requires our Navy to put all its efforts into rounding off the war readiness to maintain the constant combat alertness, and get prepared to break the enemy's will for war in contingency and carry out the military strategy of the Supreme Headquarters," Kim said.

Kim was accompanied by his daughter Ju-ae during the visit, in what marked her first public appearance since May 16 when she showed up for an on-site inspection of a preparatory committee on the North's attempt to launch a military spy satellite.

Since 2014, North Korea has commemorated Aug. 28 as Navy Day to mark the official establishment of its naval forces. Monday's visit marked Kim's first visit to the navy headquarters on Navy Day since he came to power. (Yonhap)