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.

Russia struck Kyiv with a sophisticated missile and drone attack for the first time in 73 days on Wednesday morning and damage is still being assessed, a top official in the Ukrainian capital said. Air raid warnings blared for hours.

A buildup of Russian troops in Ukraine’s southeast comes as uncertainty looms over how a Trump presidency will affect the war.

Ukrainian forces successfully repelled Russian attempts to break into the city of Kupiansk in Kharkiv Oblast, Ukrinfrom reported on Nov. 14, citing the General Staff of Ukraine's Armed Forces. Russian invaders failed to enter Kupiansk.