Defence Minister Rustem Umerov confirmed, in an interview with South Korean television, that the first armed engagements had occurred with North Korean troops in the more than 2-1/2-year-old war.

"The first battles with North Korean soldiers have opened a new chapter of instability in the world," Zelenskiy said in his nightly video address.

Zelenskiy thanked those in the world who, he said, had reacted to the North Korean troops "not just with words...but who are preparing actions to support our defence".

"We must, together with the world do everything so that this Russian step to expand the war with real escalation fails. That this step of his (Russian President Vladimir Putin) becomes a losing one -- both for him and for North Korea."

South Korea's Defence Ministry said on Tuesday that 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 that they had been engaged in combat.

Umerov, the defence minister, told South Korea's KBS television in an interview broadcast on Tuesday that there had been a "small engagement" with the North Korean troops.

"Yes, I think so. It is (an) engagement," Umerov said in English, when asked if a clash had occurred.

The report, with excerpts from the interview, quoted Umerov as saying that the engagement was small and not yet systematic in terms of mobilising soldiers.

It said Umerov told the interviewer that 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.

Plans had called for North Korean troops to undergo a month's training, he said, but that "is now being shortened to...two weeks or one week so that they could get engagement in the battlefield".

KBS quoted Umerov as saying that a total of 15,000 troops could be deployed along northeastern, eastern and southeastern parts of the 1,000-km-long (600-mile) front line in Ukraine.

Russia has declined to acknowledge that North Korean troops are on their territory, but Putin last week did not deny reports of their presence. He said it was up to Russia how to implement its defence pact with Pyongyang.