United Nations Secretary-General Antonio Guterres is "very concerned" about reports indicating that North Korean troops have been sent to Russia, and at their possible deployment to the conflict zone of Ukraine.
"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 on Sunday.
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 neighbour.
'Dangerous escalation'
US Secretary of State Antony Blinken said on Thursday that 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.
Pyongyang is widely believed to be offering military support in return for Russian nuclear technology.