President Volodymyr Zelenskiy says the first battles between the Ukrainian military and North Korean troops have opened "a new page in instability in the world".
Ukrainian Defence Minister Rustem Umerov confirmed the first engagement had occurred with North Korean troops, an apparent escalation in a conflict that began when Russia invaded Ukraine in 2022.
Zelenskiy, in his nightly video address, thanked those in the world who, he said, had reacted to the dispatch of North Korean troops to Russia in October "not just with words ... but who are preparing actions to support our defence".
"The first battles with North Korean soldiers open a new page of instability in the world," he said.
He said Ukraine, acting with the rest of the world, had to "do everything so that this Russian step to expand the war with real escalation fails".
Defence Minister Umerov told South Korea's KBS television there had been a "small engagement" with North Korean troops.
"Yes, I think so. It is (an) engagement," Umerov said when asked if a clash had occurred.
The report, with excerpts from the interview, quoted Umerov as saying the engagement was small and not yet systematic in terms of mobilising soldiers.
South Korea's Defence Ministry said more than 10,000 North Korean troops had arrived in Russia, with a "significant number" in the frontline areas, including the Kursk region, where Ukrainian forces staged an incursion in August.
Zelenskiy quoted intelligence sources as saying on Monday that 11,000 North Koreans were in Russia.
The Pentagon said at least 10,000 North Korean soldiers were in Kursk, but it could not corroborate suggestions they had been engaged in combat.
The KBS report said Umerov warned identification and other procedures would take time as the Russian military was trying to pass off the North Koreans as Buryats, a Mongolian ethnic group from Siberian regions.
Umerov said he expected a sharp rise in the number of North Koreans deployed.
"(There are) already contacts, but after a couple of weeks, we would see a more significant number and upon this, we will review it and analyse it," he said.
Expectations that North Korean troops would undergo a month's training, he said, appeared to have been shortened to one or two weeks to allow swifter deployment to the battlefield.
Russia has not acknowledged that North Korean troops are on its territory, but Putin did not deny reports of their presence.
He said it was up to Russia how to implement its defence pact with Pyongyang.
