Ukraine has lost over 40% of the territory in Russia's Kursk region that it seized in a surprise incursion in August as Russian forces have mounted waves of counter-assaults, a senior Ukrainian military source said.

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

The deal comes at a tense moment in the Ukraine conflict, with Kyiv firing British and US-supplied long-range missiles into Russia and Moscow firing a hypersonic missile at its neighbour.