Russian soldiers have punched through Ukrainian lines in Russia’s Kursk region only a day after a US official said Kyiv could hold the territory for months.

Sources from both sides confirmed the Russian breakthrough, which occurred during the heaviest fighting on Russian territory since Ukraine began its cross-border incursion in August.

DeepState, a Ukrainian military blogger, said that there was a risk that Russian forces could encircle Ukrainian forces, although they later said that the front lines in Kursk were “difficult but under control”.

“The enemy pushed through the left flank of the Kursk grouping of Ukrainian defence forces,” the blogger said. “We are on the verge of another mess due to the repetition of mistakes.”