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."

Experts tell Andy Gregory and Tom Watling that the real question is how much Kyiv’s troops are able to inflict casualties on the Russians as the price for taking the territory back

Ukrainian President Volodymyr Zelensky said Monday that Russia has deployed nearly 50,000 troops to Kursk, the southern Russian region where Kyiv launched its surprise counteroffensive in the summer.

A mass of Russian and North Korean troops is expected to target Ukrainian positions in the Kursk region of Russia.

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 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,

The video shows soldiers from Ukraine’s 36th Separate Marine Brigade targeting Russian troops and military equipment, including tanks and armored vehicles. Read also: Ukrainian forces detail enemy's failed assaults in Kursk Oblast "Russian generals send their elite soldiers to slaughter.