Ukrainian President Volodymyr Zelenskiy called on allies "not to hide" and to respond to evidence of North Korean involvement into Russia's war in Ukraine.

He said in his nightly address that Ukraine had information about the preparation of two units - possibly up to 12,000 North Korean troops - to take part in the war alongside Russian forces.

"This is a challenge, but we know how to respond to this challenge. It is important that partners do not hide from this challenge as well," Zelenskiy said.

(Ukraine President Volodymyr Zelenskiy says the West is "not to hide" from North Korea's involvement. (EPA PHOTO)

Neither North Korea nor Russia, he said, took any account of the number of dead in a conflict.

"But all of us in the world have an equal interest in ending the war, not in prolonging it. We must therefore stop Russia and its accomplices," he said.

"If North Korea can intervene in a war in Europe, then the pressure on this regime is definitely insufficient."

British Defence Secretary John Healey said on Tuesday it was "highly likely" that North Korea had begun sending hundreds of troops to help Russia in the more than two-and-a-half-year-old conflict.

A top US diplomat said on Monday that Washington was consulting with its allies on the implications of North Korean involvement and added that such a development would be a "dangerous and highly concerning development" if true.

NATO Secretary General Mark Rutte said on Monday the dispatch of North Korean troops would significantly escalate the conflict.