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.

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).

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.

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 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."

The move marks the latest reversal from the White House on what weapons it’s sending and how freely Ukraine can use them.

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 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.

Ukraine lost more than 40% of the territory in the Kursk region it controlled during a surprise operation launched in August. This happened amid waves of counterattacks organized by Russian troops, reports Reuters.

Of the territory Ukraine captured in a surprise attack in Russia's Kursk oblast in August, it has already lost 40 percent again. An army source told the Reuters news agency. “At the maximum, we had control of 1,