U.S. President Joe Biden said Ukraine should strike back against North Korean troops "if they cross into Ukraine."
NATO said on Monday thousands of North Korean troops were moving toward the front line, a development which has prompted Kyiv to call for more weapons and an international plan to keep those troops at bay.
The Pentagon confirmed on Tuesday that some North Korean soldiers are in the Kursk region, a Russian border area where Ukrainian forces staged a major incursion in August and hold hundreds of square kilometres of territory. A couple of thousand more are heading there, it said.
The United States has said any North Korean troops fighting in the war would be "fair game" for Ukrainian attacks and that Washington would not impose any fresh limits on Ukraine's use of U.S. weapons if North Korea entered the fight.
South Korea, which remains technically at war with the nuclear-armed North decades after the 1950-1953 Korean War, also condemned the deployments, with officials in Seoul worried about what Russia may be providing to Pyongyang in return.
North Korean Foreign Minister Choe Son Hui arrived in Russia's far east on Tuesday on her way to Moscow, Russian state media said. Russian state news agencies said it was not clear who Choe, making her second visit in six weeks, would meet.
