Ukraine’s human rights ombudsman has denounced the alleged execution of nine captured Ukrainian troops by Russian forces in the Kursk border region.

Dmytro Lubinets said he had written to the United Nations and the Red Cross about the allegations, accusing Moscow of breaching “all the rules and customs of war”.

The intervention follows reporting by Ukrainian battlefield analysis site DeepState, which published drone footage purporting to show the dead troops who it said were drone operators. Officials in Russia have yet to comment on the allegations.

Kyiv is believed to have deployed thousands of troops into the Russian border region since it launched its shock incursion earlier this summer.

The images published by DeepState showed the dead Ukrainian troops stripped to their underwear and lying face down in what appeared to be farmland in Kursk. The BBC cannot independently verify the images.

The outlet said the drone operators had been overrun by a rapid Russian advance.

“These actions must not go unpunished, and the enemy must bear full responsibility,” Mr Lubinets wrote in a message to Telegram. “The international community should not turn a blind eye to such crimes!”

Kyiv has frequently accused Russian of executing captured Ukrainian troops – a war crime under the Geneva Convention. Earlier this month the prosecutor general’s office alleged that Russian forces had executed 93 Ukrainian soldiers since the beginning of the conflict.

It added that an official investigation had been opened into reports that 16 Ukrainian soldiers were executed in the eastern Donetsk region near the city of Pokrovsk – where fighting has raged for months. Officials said the reports would mark the “largest mass execution” of Ukrainian prisoners of war by Russian troops since Moscow launched its invasion in February 2022.

The Kremlin denies that its soldiers have been committing war crimes in Ukraine.

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The reports come as Russian forces continue to attack Ukrainian positions in Kursk. President Volodymyr Zelensky said in his nightly address from Kyiv on Saturday that Ukrainian troops had fought off a renewed Russian advance in the region.

Analysts say that Kyiv launched the offensive to try and force Russia to redirect some of its troops from its offensive in eastern Ukraine. The Institute for the Study of War (ISW) has estimated that around 40,000 Russian forces are now active in Kursk – up from 11,000 when Ukrainian troops first crossed the border.

But the offensive has failed to slow Russian momentum in the eastern Donbas region, where relentless attacks has slowly pushed Ukrainian forces backwards.

The Ukrainian leader acknowledged that “there are very difficult conditions, with harsh enemy actions” in both Donetsk and Zaporizhzhia in his address on Saturday.

On Sunday morning, the Russian defence ministry said its forces had seized the village of Mykhailivka, which sits along a highway near the key city of Pokrovsk.

Russian forces have been advancing towards Pokrovsk – which is a key logistics hub – for months. Experts say if Russia can seize the city Ukraine’s ability to resupply units in other crucial towns would become far more difficult.

Meanwhile, Russian aerial attacks on Ukraine continued overnight. Air force officials in Kyiv said Moscow launched 68 drones and four missiles towards Ukrainian territory.