Traditional allies Russia and North Korea have drawn closer since Moscow's 2022 invasion of Ukraine, with Seoul accusing Kim of sending thousands of troops and container loads of weapons to help Moscow fight Kyiv.

Neither Moscow nor Pyongyang has officially confirmed the troop deployment, but the two countries signed a sweeping military deal last year, including a mutual defence clause, when Russian President Vladimir Putin made a rare visit to the nuclear-armed North.

Moscow's top security official Sergei Shoigu was visiting North Korea on Friday, Russian news agency TASS reported, the latest in a recent series of high-level exchanges between North Korea and Russia.

Shoigu passed on "greetings and best wishes" from Putin to Kim in a meeting with the North Korean leader.

He also hailed a strategic partnership treaty that includes mutual defence components, signed and ratified last year, as "fully meeting the interests of both countries".

Kim oversaw the test-fire of the country's latest anti-aircraft missile system, the Korean Central News Agency (KCNA) said Friday, without specifying when the test happened.

Kim said it showed North Korea's army would be "equipped with another major defence weapon system with laudable combat performance," KCNA said.

The announcement came a day after South Korea wrapped up a major annual joint military exercise with the United States, known as Freedom Shield, which Pyongyang separately denounced Friday as "a rehearsal of war of aggression".

The North -- which attacked its neighbour in 1950, triggering the Korean War -- has long been infuriated by any military exercises between the United States and the South.

Seoul said last week the North had fired "multiple unidentified ballistic missiles" after it began the joint drills, which involve US soldiers stationed in South Korea.

Pyongyang also carried out a test-launch of strategic cruise missiles in the Yellow Sea in late February, which it said showed off "counterattack capabilities".

The most recent Freedom Shield exercise featured a collaborative drill focused on countering weapons of mass destruction, specifically targeting nuclear, chemical, biological, and radioactive threats.

Relations between Pyongyang and Seoul have been at one of their lowest points in years, with the North launching a flurry of ballistic missiles last year in violation of UN sanctions.

- Tests for Russia? -

Ahn Chan-il, a defector-turned-researcher who runs the World Institute for North Korea Studies, told AFP that the latest launch appears to be "the testing of weapons for export to Russia to be used in Ukraine".

Pyongyang is using the US-South Korea joint exercises as an excuse to develop and export such weapons to Moscow, he added.

South Korean and Western intelligence agencies have said that more than 10,000 soldiers from the reclusive North were sent to Russia last year to help it fight a shock Ukrainian offensive into the Kursk border region.

Seoul's spy agency said last month that the North has sent more soldiers and re-deployed several to the frontline in Kursk.

Seoul has also accused the North of sending container loads of weapons to Russia, and a recent report by Seoul's defence ministry said the North was "continuing to provide weapons, ammunition and other military support to Russia following its troop deployment in the Ukraine war".

North Korea's repeated weapons tests are aimed as "showcasing the development of strategically valuable weapons," Hong Min, a senior analyst at the Korea Institute for National Unification, told AFP.

"If North Korea improves its missile interception and air defense capabilities through technological cooperation with Russia... it could significantly affect South Korea's 'kill chain'" defence plan, he said.

"In particular, North Korea may receive assistance for its vulnerable radar systems and field command vehicles from Russia," he added.