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.

KYIV (Reuters) -Blasts rang out over Kyiv on Wednesday morning after officials said Russia launched its first missile attack on the Ukrainian capital since August and urged residents to take cover. The scale of the strike and extent of any damage were not immediately clear.