Kyiv's forces struck an advanced Russian air-defense system in the border Kursk region, according to Ukraine's military, shortly after the U.S. green-lit attacks over the border using long-range American weapons and as North Korean soldiers are expected to join front-line clashes in the area.

Ukraine has lost more than 40 per cent of the territory in Russia’s Kursk region that it seized in August, a senior Ukrainian military source said.