DeepState, a Ukrainian battlefield analysis site close to Ukraine's defence ministry, alleges the prisoners of war were shot on 10 October in the Russian region of Kursk, where Kyiv launched an incursion in August.

Ukraine's human rights ombudsman has urged international organisations to respond.

Dmytro Lubinets said on Telegram that he sent letters to the United Nations and the International Committee of the Red Cross regarding the claim, calling it "another crime committed by the Russians".

Earlier this month, the Prosecutor General's Office of Ukraine said Russian troops had killed 16 captured Ukrainian soldiers in the partially occupied Donetsk region.

There was no immediate response from Russian officials.

Meanwhile, the Ukrainian air force said on Sunday that its air defences had shot down 31 of 68 drones launched at Ukraine by Russia overnight in the regions of Kyiv, Poltava, Chernihiv, Sumy and Cherkasy.

A further 36 drones were "lost" over various areas, it said, probably having been electronically jammed.

The air force added that ballistic missiles struck Odesa and Poltava while Chernihiv and Sumy came under attack by a guided air missile. Local authorities did not report any casualties or damage.

Ukrainian President Volodymyr Zelenskyy said on Sunday that Russia had launched around 900 guided aerial bombs, more than 40 missiles and 400 drones against Ukraine over the past week.

Mr Zelenskyy appealed on social media platform X to Ukraine's allies to "provide the necessary quantity and quality of air defence systems" and "make decisions for our sufficient range".

Kyiv is still awaiting word from its Western partners on its repeated requests to use the long-range weapons they provide to hit targets on Russian soil.

In Russia, the defence ministry said that 13 Ukrainian drones were shot down over three regions of Russia: six each in the Belgorod and Kursk regions, and one in the Bryansk region, all of which border Ukraine.