“The secretary-general is very concerned about reports of troops from the Democratic People’s Republic of Korea being sent to the Russian Federation, including their possible deployment to the conflict zone,” said Stephane Dujarric, the UN chief’s spokesman.

US intelligence has said North Korean forces have made their way to Russia’s Kursk border region, with Washington and Seoul urging Pyongyang to withdraw its troops.

North Korea and Russia have not denied the troop deployment reports, with the grinding war in Ukraine still ongoing more than 2.5 years after Moscow invaded its neighbor.

US secretary of state Antony Blinken said Thursday the deployment of North Korean troops against Ukrainian forces could happen “in the coming days.”

On Sunday, Guterres said such a deployment would be “a very dangerous escalation” of the war in Ukraine.

“Everything must be done to avoid any internationalisation of this conflict,” he said, while reiterating a call for “meaningful efforts” to end the war.

North Korea and Iran have emerged as Russia’s main backers in Ukraine, with both believed to be supplying Moscow with military hardware.

Pyongyang is widely believed to be offering military support in return for Russian nuclear technology.