Ukrainian President Volodymyr Zelenskiy has warned that Russia intends to deploy troops from North Korea in combat areas as early as October 27.
He also called for "tangible" international pressure on the two countries to get them to respect UN legislation.
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Ukraine's military intelligence service said on October 24 that it had detected the presence of North Korean troops deployed in Russia’s Kursk region, where the first units arrived on October 23.
Ukrainian forces staged a major incursion in August in Kursk, a Russian region bordering Ukraine, and are holding territory there.
Kyiv says this territory is being used as a buffer zone against Russian strikes on Ukraine.
According to intelligence, Zelenskiy said on X on October 25, "the first North Korean soldiers are expected to be deployed by Russia to combat zones as early as October 27-28. He called the move "a clear escalation by Russia."
"The world can clearly see Russia's true intentions: to continue the war," he said. "This is why a principled and strong response from global leaders is essential."
White House national security spokesman John Kirby said it's possible that there are now more than 3,000 troops from North Korea that have been dispatched to Russia for training.
Kirby said on a call with reporters on October 25 that it was possible that some of the North Korean troops would be deployed to the Kursk region but cautioned that he did not know for what purpose.
The top national-security advisers for the United States, Japan, and South Korea had met, Kirby said, adding that they "expressed grave concern" about the possible deployment of North Korean troops on the battlefield against Ukraine.
The three advisers called on Russia and North Korea "to cease these actions that only serve to expand the security implications of Russia's brutal and illegal war beyond Europe and into the Indo-Pacific," Kirby said.
U.S. Secretary of Defense Lloyd Austin said on October 23 that evidence of North Korea sending troops to Russia was the "next step" after Pyongyang provided Moscow with arms.
Analysts are still assessing the situation, Austin said, but Pyongyang, which has denied the accusations, could face consequences for aiding Russia directly.
SEE ALSO: Today's Autocracies Are Networked In Efforts To Erode Democracy, Says Author Anne ApplebaumAsked about whether any North Korean troops were in Russia, Russian President Vladimir Putin neither confirmed nor denied the claim.
When asked about satellite photos of troops at a news conference after a summit in Russia's Kazan, Putin said, "images are a serious thing, if there are images, then they reflect something."