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."

A week after Ukrainian soldiers clashed with North Korean troops for the first time, 50,000 Russian and North Korean troops are expected to target Ukrainian positions in the Kursk region of Russia in an effort to reclaim some of its territory.

Russia has amassed a large force of tens of thousands — including recently arrived North Korean troops — to carry out an assault on the Ukrainian positions in Russia’s Kursk region expected in the coming days,