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.

KYIV, Ukraine >> 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 counterassaults, a senior Ukrainian military source said.

Ukraine has now lost more than 40% of the territory of Kursk Oblast, which it has controlled since August, as a result of Russian counterattacks, says a Reuters source in the General Staff of the Ukrainian Armed Forces.

The Ukrainian source revealed that their forces initially controlled up to 1,376 square kilometres (531 square miles) of territory in Kursk but now hold around 800 square kilometres (309 square miles).

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.

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.