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.

Last week, Pyongyang said it had also conducted its first test of a new generation of strategic cruise missiles.

"It is believed that North Korea has commenced mass production of cruise missiles ordered by Russia," Ahn Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told AFP.

Washington and Seoul have accused the North of supplying Moscow with weapons for use in Ukraine, despite a raft of UN sanctions banning any such arrangements.

"It looks like they are conducting ... experiments of these (ordered) missiles at sea, causing disruption to South Korea and the United States," Anh said, adding that all guided missiles needed to undergo a minimum of five tests before being deployed on the battlefield.

- South Korean, US elections -

In December, Seoul's spy agency issued a statement forecasting that Pyongyang would carry out military and cyber provocations in 2024, targeting election campaigns in the United States and South Korea.

North Korean leader Kim late last year instructed his aides to "come up with measures to cause a big stir in South Korea early next year", according to the statement by Seoul's spy agency.

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 North's official Korean Central News Agency said.

"North Korea seems to indirectly support former US President Donald Trump by emphasising the shortcomings of South Korea and (US President Joe Biden's administration) policy towards North Korea by increasing tensions on the Korean Peninsula," Yang Moo-jin, president of the University of North Korean Studies, told AFP.

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 on January 25.