Seoul's Joint Chiefs of Staff said the military had "detected at around 11 am (0200 GMT) the firing of multiple unidentified cruise missiles" into the sea off the country's west coast.

The South Korean military has "stepped up surveillance in close coordination with the United States", the JCS said in the statement.

The military is "closely monitoring for signs of additional activity" by the North Korean army, it said, adding they were "closely analysing" the launch.

The Friday launch comes three days after the nuclear-armed North conducted what it said was a test-fire of a strategic cruise missile.

On Sunday, Kim also oversaw the launch of two Pulhwasal-3-31s, a new type of strategic cruise missile, purportedly from a North Korean submarine.

Pyongyang has also claimed this month to have tested an "underwater nuclear weapon system" and a solid-fuelled hypersonic ballistic missile.

Unlike their ballistic counterparts, the testing of cruise missiles is not banned under current UN sanctions on Pyongyang.

Cruise missiles tend to be jet-propelled and fly at a lower altitude than more sophisticated ballistic missiles, making them harder to detect and intercept.

Analysts have warned that North Korea could be testing cruise missiles ahead of sending them to Russia for use in Ukraine, with Washington and Seoul claiming Kim has shipped weapons to Moscow as part of a possible illicit deal, banned by rafts of UN sanctions.

The latest launch follows Kim Jong Un inspecting warships at a naval shipyard as he looks to bolster his maritime forces as part of "war preparations", state media said Friday.

In recent weeks, Kim has declared South Korea his country's "principal enemy", jettisoned agencies dedicated to reunification and outreach, and threatened war over "even 0.001 mm" of territorial infringement.

"The strengthening of the naval force presents itself as the most important issue in reliably defending the maritime sovereignty of the country and stepping up the war preparations at present," Kim said at the Nampho Dockyard, according to the Korean Central News Agency.

A nuclear-powered submarine was on Kim's strategic weapons wish list set out at a key party congress in 2021, along with a hypersonic warhead, spy satellites and solid-fuel intercontinental ballistic missiles.

During his visit to the dockyard, Kim was briefed on various warships under construction as well as preparations for a "new huge plan" assigned by the ruling party.

No details of the plan were provided.

Kim "expressed expectation that the workers of the dockyard would successfully build on the world level the major warships," KCNA said.

North Korea last year launched what it called its first "tactical nuclear attack submarine", which Seoul's military at the time said did not look to be operational.

Analysts said the vessel appeared to be modified from an existing diesel-electric submarine originally designed in the 1950s, and have posed questions about its limitations and vulnerabilities as a platform.

The US stations about 30,000 troops in the South to help it fend off military threats from Pyongyang.