So far this year, 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.

Pyongyang has also ramped up weapons tests, including cruise missiles, an "underwater nuclear weapon system" and a solid-fuelled hypersonic ballistic missile.

"Our military detected several unknown cruise missiles over the waters northeast of Wonsan around 09:00 today (1200 GMT) and South Korea-US intelligence authorities are conducting a detailed analysis," the Joint Chiefs of Staff said in a statement.

It added that the military was "strengthening surveillance and vigilance, and closely watching for any additional signs and activities from North Korea".

Earlier this week, North Korea announced it had tested a new control system for a multiple rocket launcher it said would have an "increased" battlefield role.

The cruise missile launches have prompted speculation from experts that it is testing the weapons before shipping them to Moscow for use in Ukraine.

Pyongyang and Moscow have bolstered ties in recent months, with leader Kim Jong Un making a rare trip to Russia to meet President Vladimir Putin in September.

Seoul and Washington have accused the North of supplying Russia with weapons in exchange for technical support for Kim's nascent satellite programme, which would violate a raft of UN sanctions on both regimes.

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.

"It is believed that North Korea exported large quantities of multiple rocket launchers to Russia last year," Ahn Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told AFP.

He added that there may have been quality issues with some of the exported weapons and that the recent spate of testing could be leader Kim "taking action to address the issue".

- 'Put an end' to South Korea -

Kim repeated last week that Pyongyang would not hesitate to "put an end" to South Korea if attacked, calling Seoul the North's "most dangerous and first enemy state and invariable arch-enemy".

In January, North Korea fired an artillery barrage near two South Korean border islands, prompting a live-fire drill by the South and evacuation orders for residents.

South Korean President Yoon Suk Yeol has vowed a strong response if Pyongyang attacks, calling on his military to "act first, report later" if provoked.

The hawkish Yoon has bolstered defence cooperation with the United States and Japan since coming to office in 2022, including expanding joint drills, to counter Pyongyang's growing threats.

With a US presidential election later this year, the North could potentially ramp up its provocations to take advantage of American political gridlock, Cheong Seong-chang, director of the Center for Korean Peninsula Strategy at the Sejong Institute, wrote in a report.

North Korea could "conduct attacks on (South Korea's) Baengnyeong Island, Daecheong Island, and Socheong Island based on their confidence in the advancement of nuclear and missile capabilities," he wrote.

Pyongyang has drawn closer to Moscow in areas beyond defence, with a group of Russian tourists -- the first known foreign tour group since before pandemic-linked border closures in 2020 -- arriving in the North Friday for a four-day visit.

It has become harder for Russians to travel to Europe and the United States since sanctions were imposed following the invasion of Ukraine. Russian Foreign Minister Sergei Lavrov, who has also visited Pyongyang, said last year that the North could be recommended as a tourist destination, Tass reported.