This month, Pyongyang has conducted tests of what it called an "underwater nuclear weapon system", a solid-fuelled hypersonic ballistic missile, and a new generation of strategic cruise missiles.

Relations between the two Koreas have sharply deteriorated, with Kim Jong Un declaring Seoul his principal enemy, as he pulls closer to Moscow, including, Washington says, sending weapons for use in Ukraine.

South Korea's military said it had detected the launch of several cruise missiles early Tuesday, adding that it was "conducting a detailed analysis" while strengthening surveillance in cooperation with ally the United States.

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.

Despite rafts of UN sanctions, Seoul and Washington say Kim has been shipping weapons to Russia, possibly in exchange for Moscow's technical assistance for Pyongyang's budding spy satellite programme.

Kim made a rare overseas trip to Russia in September to meet President Vladimir Putin at a cosmodrome, with Putin now set to pay a visit to Pyongyang in return. The North successfully put its first spy satellite into orbit in November.

"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.

"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.

Hong Min, a senior analyst at the Korea Institute for National Unification in Seoul, said one cannot "rule out the possibility" that North Korea is conducting test-fires of cruise missiles intended for export to Russia.

"During the Ukraine war, cruise missiles have played a significant role for Russia in targeting strategic facilities in Ukraine," he told AFP.

Chun In-bum, a retired South Korean army general, added that "North Korean weapons are for sale as long as the price is right."

- Election disruption? -

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.

Thae Yong-ho, a former North Korean diplomat who defected to the South, said this month that Kim was not likely looking to trigger a war, given that Pyongyang was selling a "significant number" of weapons to Russia for its war in Ukraine.

"If Kim Jong Un intends to initiate a war this year, does it make sense for him to send a substantial quantity of his weapons to Russia in containers?" Thae said in an interview with The Chosun Daily, adding that the North was aiming to deter Seoul and Washington by "creating the impression of a significant impending action".

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.