South Korea's military "detected several unknown cruise missiles launched into the West Sea of North Korea around 07:00 (2200 GMT)", Seoul's Joint Chiefs of Staff said in a statement.

South Korean and US intelligence agencies "are conducting a detailed analysis," the JCS said.

"Our military is cooperating closely with the U.S. while strengthening surveillance and vigilance, and is closely monitoring North Korea's activities," it added.

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.

Recent months have seen a sharp deterioration in ties between the two Koreas, with both sides jettisoning key tension-reducing agreements, ramping up frontier security, and conducting live-fire drills along the border.

Pyongyang has accelerated weapons testing in the new year, including tests of what it called an "underwater nuclear weapon system" and a solid-fuelled hypersonic ballistic missile.

On Monday, state media said North Korean leader Kim Jong Un had overseen the test launch of a new strategic cruise missile from a submarine.

Photos showed a missile soaring into the sky from the water, leaving a huge trail of white smoke, but it was not clear if it had been fired from a submarine.

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

He also said Pyongyang would not recognise the two countries' de facto maritime border, the Northern Limit Line, and called for constitutional changes allowing the North to "occupy" Seoul in war, the Korean Central News Agency said.

- 'Overwhelming response' -

In Seoul, President Yoon Suk Yeol told his cabinet that should the nuclear-armed North carry out a provocation, South Korea would hit back with a response "multiple times stronger", pointing to his military's "overwhelming response capabilities".

Pyongyang's latest launch comes after South Korea conducted a 10-day special forces infiltration drill off the country's east coast, "in light of serious security situations" with the North, which ended January 25.

At North Korea's year-end policy meetings, Kim threatened a nuclear attack on the South and called for a build-up of his country's military arsenal ahead of armed conflict he warned could "break out any time".

Earlier this month, the North launched a solid-fuel hypersonic missile, just days after Pyongyang staged live-fire exercises near the country's tense maritime border with South Korea, which prompted counter-exercises and evacuation orders for some border islands belonging to the South.

Kim also successfully put a spy satellite into orbit late last year, after receiving what Seoul said was Russian help, in exchange for arms transfers for Moscow's war in Ukraine.