South Korean President Yoon Suk Yeol has warned the world must be wary of the growing alliance between Russia and North Korea.
It follows a high-profile visit to Russia by North Korean leader Kim Jong-un, where he met Russian President Vladimir Putin to discuss increasing military co-operation.
Nineteen months into the Russia-Ukraine war, Moscow has turned to close allies as its forces have burnt through millions of artillery shells and rockets, while Ukraine has also urged western backers to accelerate artillery production.
North Korea has over the decades built up a vast arsenal of artillery, much of it compatible with Russian systems.
North Korea is, however, subject to a 2006 UN arms embargo that bans Pyongyang from exporting or importing weapons.
“Military co-operation between North Korea and Russia is illegal and unjust as it contravenes UN Security Council resolutions and various other international sanctions,” Mr Yoon said in written responses to questions from the Associated Press before his departure to New York to attend the UN General Assembly.
North Korean state media highlighted the predominant focus of Mr Kim's trip, describing his discussions with Russian Defence Minister Sergey Shoigu to expand "strategic and tactical co-ordination" between their countries' militaries, as western concerns grow about an arms alliance that could help Russian President Vladimir Putin with the war in Ukraine.
The meeting between the two leaders also signalled expanding co-operation into other sectors, besides defence.
Russia’s RIA Novosti state news agency released videos of Mr Kim – dressed in a black suit and accompanied by senior North Korean officials – talking to Russian authorities through translators as he walked through the campus of the Far Eastern Federal University on Russky Island.
Mr Kim met Oleg Kozhemyako, governor of Russia’s Primorye region, which includes the port city of Vladivostok, as well as Natural Resources Minister Alexander Kozlov and the university’s president Boris Korobets, Russian state media reported.
An online video posted by Mr Kozlov showed Mr Kim being shielded from light rain as he was shown around the campus. Mr Kozlov told Mr Kim that North Koreans were studying there and that the school would be glad to have more students from his country studying hydropower, state media said.
Mr Kozhemyako had earlier said he would discuss with Mr Kim exchange programmes for schoolchildren to attend summer camps in each other’s country.
Mr Kim was later seen at the island’s Primorsky Aquarium, Russia's largest, where he watched performances featuring beluga whales, bottlenose dolphins, fur seals and Misha the walrus, which he seemed to particularly enjoy, according to Russian state media.
A day after visiting an aircraft plant in Komsomolsk-on-Amur that produces Russia’s most powerful fighter jets, Mr Kim on Saturday travelled to an airport near Vladivostok, where Mr Shoigu and other senior military officials showed off strategic bombers and other warplanes.
All the Russian aircraft shown to Mr Kim were among the types that have been in active use in the war in Ukraine, including the Tu-160, Tu-95 and Tu-22 bombers that have launched cruise missiles.
