Kim inspected one of his fleets in the East Sea, also known as the Sea of Japan, and watched as the crew staged a drill launching "strategic cruise missiles," state-run news agency KCNA reported.

The drill "aimed to reconfirm the combat function of the ship and the feature of its missile system and make the seamen skilled at carrying out the attack mission in actual war," the report said.

It did not say when the visit took place, or give further details on the types of missiles launched -- though it said they "rapidly hit target without even an error".

But Seoul said the KCNA report was "exaggerated and contained many discrepancies from the truth".

"Both South Korea and the United States had been monitoring any related signs -- which we detected in advance -- in real-time," Seoul's Joint Chiefs of Staff said in a statement.

North Korea's announcement came as the annual Ulchi Freedom Shield drills, a major joint exercise between Seoul and Washington, kicked off Monday.

The exercises, which are aimed at countering growing threats from the nuclear-armed North, will run through August 31.

Pyongyang views all such exercises as rehearsals for an invasion and has repeatedly warned it would take "overwhelming" action in response to the drills.

Suspected North Korean hackers have already targeted the exercises, with email attacks on South Korean contractors working at the allies' combined exercise war simulation centre, police have said.

But "our military will continue to maintain a firm preparedness posture... conducting joint exercises and training with high intensity and thoroughness, and being able to overwhelmingly respond to any provocations by North Korea," the JCS said Monday.

- 'New chapter' -

The announcement of the cruise missile test also comes days after US President Joe Biden hosted South Korean leader Yoon Suk Yeol -- along with Japanese Prime Minister Fumio Kishida -- at Camp David.

At a press conference on Friday, the leaders said they saw a "new chapter" of close three-way security cooperation after the summit, which would have been unthinkable until recently due to the legacy of Japan's harsh 1910-1945 occupation of the Korean peninsula.

It was the first time the three countries' leaders have met for a standalone summit, and while China was a main topic, they also discussed North Korea.

The three leaders agreed to a multi-year plan of regular exercises in all domains, going beyond one-off drills in response to Pyongyang, and made a formal "commitment to consult" during crises, with Biden saying they would open a hotline.

The leaders also agreed to share real-time data on North Korea and to hold summits every year.

"Deepening US-Japan-South Korea trilateral cooperation can encourage diplomacy with Pyongyang by pressuring China to pressure North Korea to return to talks," said Leif-Eric Easley, a professor at Ewha University in Seoul.

"Until that happens, the Kim regime will continue amping up its nuclear threats and launching missiles."