It is confirmed: They are at war

"Today I can confirm that more than 10,000 DPRK troops have been deployed to eastern Russia, and most of them have been moved to the western Kursk region, where they have begun to engage in combat operations with Russian forces," said Vedant Patel, a spokesman for the US State Department at the regular briefing.

He said Washington was "very concerned" by the development, which came barely a month after a South Korean intelligence report said Russian naval ships had transported tens of thousands of North Korean soldiers to training camps in the Russian Far East.

"Russian forces trained DPRK soldiers in artillery, drone operations and basic infantry skills, including trench cleaning, which are critical skills for frontline operations," Patel said.

"However, Russia's success on the battlefield with these DPRK soldiers will largely depend on how well the Russians manage to integrate them into their military."