Blinken said that of the 10,000 North Korean troops believed by the United States to have entered Russia, up to 8,000 "have been deployed to the Kursk region" bordering Ukraine.

"We've not yet seen these troops deploy into combat against Ukrainian forces, but we would expect that to happen in the coming days," Blinken told a news conference after US-South Korea talks.

Russia has been training North Korean troops "in artillery, UAVs, basic infantry operations, including trench clearing, indicating that they fully intend to use these forces in frontline operations," he said.

"Should these troops engage in combat or combat support operations against Ukraine, they would become legitimate military targets," Blinken said.