The U.S. says there will be a "firm response" as Ukrainian forces holding ground in Russia's Kursk region now face North Korean troops "engaging in combat."

Russia attacked the Ukrainian capital Kyiv with a sophisticated combination of missiles and drones for the first time in 73 days on Wednesday, authorities reported, as the Pentagon said most of the North Korean troops sent to help Moscow’s war effort are fighting to drive Ukraine’s army off Russian soil in the Kursk border region.

North Korean soldiers have engaged in combat operations against Ukraine alongside their Russian allies, South Korea's spy agency said on Wednesday. Seoul's confirmation comes amid growing global criticism as the two countries strengthen military ties,