South Korea’s president sounded a warning to fellow world leaders on Wednesday about possible co-operation between North Korea and Russia.

Speaking before the UN General Assembly in New York, Yoon Suk Yeol said any action by a permanent UN Security Council member to circumvent international norms would be dangerous and “paradoxical”.

He remarked on North Korean leader Kim Jong Un’s visit last week to Russia, which is one of the five permanent members of the council, the UN’s most powerful body.

Mr Kim met Russian President Vladimir Putin in Russia’s far east. The two said they may co-operate on defence issues but gave no specifics, which left South Korea and its allies — including the US — uneasy.

“It is paradoxical that a permanent member of the UN Security Council, entrusted as the ultimate guardian of world peace, would wage war by invading another sovereign nation and receive arms and ammunition from a regime that blatantly violates Security Council resolutions,” Mr Yoon told fellow leaders on the second day of the UN General Assembly’s annual gathering of leaders.

He had been expected to raise the issue.

Mr Yoon said that if North Korea “acquires the information and technology necessary” to enhance its weapons of mass destruction in exchange for giving conventional weapons to Russia, that would also be unacceptable to the South.

“Such a deal between Russia and the DPRK will be a direct provocation threatening the peace and security of not only Ukraine but also the Republic of Korea,” he said, using the acronym for North Korea’s official name, the Democratic People’s Republic of Korea.

“The Republic of Korea, together with its allies and partners, will not stand idly by.”

South Korea has expressed support for Ukraine, in its fight against the 2022 Russian invasion of its territory. At the G20 summit in India earlier this month, Mr Yoon said Seoul would contribute 300 million dollars (£242 million) to Ukraine next year and — eventually — a support package worth more than two billion dollars (£1.6 billion).

“The nuclear and missile programmes of the Democratic People’s Republic of Korea pose not only a direct and existential threat to the peace of the Republic of Korea, but (are) also a serious challenge to peace in the Indo-Pacific region and across the globe,” Mr Yoon said in his speech.

Foreign experts speculate that Russia and North Korea were pushing to reach arms transfer deals in violation of Security Council resolutions. Both countries are in major disputes with the West, and both are under international sanctions.

While Russian-North Korean co-operation is feared to fuel Russia’s war efforts in Ukraine, it has also encouraged unease in South Korea, where many think a Russian transfer of sophisticated weapons technologies would help North Korea acquire a functioning spy satellite, a nuclear-powered submarine and more powerful missiles.

On Tuesday, South Korean Vice Foreign Minister Chang Ho-jin summoned the Russian ambassador to Seoul, Andrey Kulik, and urged Moscow to immediately stop its military co-operation with North Korea, which he said would have a “very negative impact” on its relations with the South.

North Korea has been increasing its nuclear arsenal for years, ratcheting up tensions in the region as it threatens to use nuclear weapons in conflicts. It regularly conducts missile tests, particularly in the past year.

In response, Yoon and U.S. President Joe Biden in April agreed to expand joint military exercises, increase the temporary deployments of U.S. strategic assets and launch a bilateral nuclear consultative group.