SEOUL, South Korea -- Kim Jong Un has called for boosting North Korea's navy, saying the country's waters brimmed with "the danger of a nuclear war," state media reported Tuesday, as Seoul, Washington and Tokyo carried out joint naval drills.

North Korean leader Kim Jong Un (STR KCNA via KNS AFP)

Kim slammed growing trilateral cooperation between the "gang bosses" of the United States, South Korea and Japan, saying they had recently "closeted with each other", the official Korean Central News Agency reported, in an apparent reference this month's Camp David summit.

He accused Washington of being "more frantic than ever before" by conducting joint naval exercises and deploying nuclear strategic assets in the waters around the Korean peninsula on a permanent basis, the report said.

"Owing to the reckless confrontational moves of the US and other hostile forces, the waters off the Korean Peninsula have been reduced into the world's biggest war hardware concentration spot, the most unstable waters with the danger of a nuclear war," Kim said, according to KCNA.

"To achieve the successes in rapidly developing the naval force has become a very urgent issue in view of the enemies' recent aggressive attempts."

Kim acknowledged that the North's navy "had not been armed with up-to-date weapons and combat equipment" but said that even so, it had managed "great achievements of more weighty significance" than the country's better-funded army.

He also promised that the navy would be given new weapons as part of North Korea's policy for "expanding the tactical nuclear weapons operation".

The navy will become a "component of the state nuclear deterrence," he added.

'Very weak' navy 

Photos carried by the official Rodong Sinmun newspaper showed Kim, accompanied by his young daughter, inspecting the navy command and taking photos with scores of naval officers.

"South Korea, the United States and Japan agreed to strengthen military cooperation and decided to conduct joint military exercises on a regular basis at the Camp David summit," Cheong Seong-chang, researcher at the Sejong Institute, told AFP.

"Therefore, North Korea may feel an urgent need to strengthen its naval power," he said, adding that some intelligence reports suggested Russia and the North were considering holding joint naval training.

"For that North Korea would need a naval vessel but the country's navy is very weak, so I think Kim Jong Un is showing off his will to strengthen the naval forces," Cheong added.

The United States, South Korea and Japan held a joint naval missile defense exercise on Tuesday to counter Pyongyang's growing nuclear and missile threats.

The exercise in international waters off South Korea's southern island of Jeju involved destroyers equipped with Aegis radar systems from the three countries, the South Korean navy said in a statement.

Tuesday's trilateral naval exercises marked the first such drills since the Camp David summit and followed similar ones in July, April, and February this year, according to the Yonhap news agency.

Seoul, Washington and Japan have beefed up their defense cooperation in recent months in response to increasing missile provocations by the North.

The United States and South Korea are also holding their annual Ulchi Freedom Shield exercises, which always infuriate Pyongyang.

North Korea has conducted a record number of weapons tests this year, and last week carried out its second attempt to put a spy satellite into orbit, although it ended in failure.

Kim has declared North Korea an "irreversible" nuclear power and has called for ramped-up arms production, including tactical nuclear weapons.