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.

The Russian counteroffensive in western Russia’s Kursk Oblast is off to a bloody and chaotic start. After six days, Ukrainian drone operator Kriegsforscher has identified 77 destroyed Russian armored vehicles along the northwestern edge of the 250-square-mile salient Ukrainian troops occupy in Kursk.