Ukrainian President Volodymyr Zelenskyy said on Friday that Russia plans to deploy North Korean troops to the battlefield starting Oct. 27-28, citing intelligence reports, Paralel.Az reports citing Reuters.

"According to intelligence, the first North Korean soldiers are expected to be deployed by Russia to combat zones as early as October 27-28. This is a clear escalation by Russia," Zelenskyy said on X after receiving reports from his top commander.

Zelenskyy did not say which frontline sector North Korean soldiers are expected to be sent to or give any other details.

Ukrainian military intelligence said on Thursday that the first North Korean units had already been recorded in Russia's Kursk border region, where the Ukrainian military has been operating since staging a major incursion in August.

Around 12,000 North Korean troops, including 500 officers and three generals, were already in Russia, and training was taking place on five military bases, it said.

The United States said that it had seen evidence of North Korean troops in Russia, and South Korean lawmakers said about 3,000 soldiers had been sent to support the Kremlin's war in Ukraine, with more to follow.

Russian President Vladimir Putin said on Thursday that it was Moscow's business how to implement a partnership treaty with Pyongyang which he said included a mutual assistance clause for each side to help the other repel external aggression.

Putin did not deny that North Korean troops were in Russia. "This is our business," he said.

Western allies have said their involvement in the Ukraine conflict would be a major escalation. Zelenskyy has said that is not enough.

"North Korea's actual involvement in combat should not be met with indifference or uncertain commentary, but with tangible pressure on both Moscow and Pyongyang," he added.

Ukrainian prosecutors said on Friday they had launched an investigation into North Korea's support for Russia in the war as a possible crime of aggression.