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.

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. Russia had deployed 59,000 troops to the region since the surprise invasion by Kyiv’s forces,

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

Russia said on Friday that Ukraine had returned 46 Russian citizens who were taken there after Ukrainian forces seized a chunk of Russia's western Kursk region in August.

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.

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.

Britain allowed Ukrainians to use Storm Shadows a week after President Biden authorized the use of American-made missiles inside Russia, a shift from a more cautious military strategy.

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."