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

The Kursk offensive was the first ground invasion of Russia by a foreign power since World War Two and caught Moscow unprepared.

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.