Ukraine says an estimated 11,000 North Korean soldiers were in the Kursk border region, where Ukrainian troops have a foothold.
In recent weeks, South Korean and US intelligence as well as Nato have said that they have seen evidence of North Korean troops being involved in Russia’s war.
But Moscow and Pyongyang have so far not responded directly to any of the allegations.
Ukraine’s top counter-disinformation official Andriy Kovalenko first said in a Telegram post on Monday that North Korea’s “first military units… [had] already come under fire in Kursk”.
In an interview with South Korean broadcaster KBS, Rustem Umerov confirmed this, saying he expects a “significant number” of the North Korean troops to be engaged in combat, though he added it was “so far just small contacts, not full-scale engagement”.
Most of them are still undergoing training, he added.
“They’re wearing Russian uniforms, they’re undergoing tactical training, and they’re being deployed under various commands of the Russian army on the front lines,” Umerov said.
He said it was likely that five units, each consisting of around 3,000 soldiers, would be deployed across the battlefield.
He did not mention if there were any casualties.
In a daily video address on Wednesday, Zelensky called on Ukraine “together with the world… [to do] everything to make this Russian step toward expanding the war… a failure. Both for them, and for North Korea.”
Reports of such a move by North Korea have also alarmed the South, raising tensions between the two sides.
Late last month, Seoul had summoned Russia’s ambassador, seeking the “immediate withdrawal” of North Korean troops from Ukraine. It also warned that it was considering directly supplying arms to Ukraine.
Analysts have said that Pyongyang could be paid, or may be given access to Russian military technology in exchange for the troops.
On Wednesday, Russian lawmakers will vote to ratify a mutual defence treaty with North Korea, first proposed during Russian President Vladimir Putin’s lavish visit to Pyongyang in June.
It pledges that Russia and North Korea will help each other in the event of “aggression” against either country.