Moscow (AFP) - Russia threatened Kyiv on Wednesday it would respond “extremely” harshly to all future incursions, after Moscow deployed jets and artillery to fight off an armed group that crossed over from Ukraine.

As Russia took stock following the most serious attack on its soil since Moscow’s offensive in Ukraine began in February 2022, the chief of the Wagner mercenary group said 10,000 prisoners he recruited died in Ukraine.

“We will continue to respond promptly and extremely harshly to such actions by Ukrainian militants,” Russian Defence Minister Sergei Shoigu told military officials after two days of fighting in the southern region of Belgorod.

On Wednesday, Belgorod governor Vyacheslav Gladkov said the territory was targeted overnight by numerous drones. Regional authorities said 13 people had been injured as the area came under sustained artillery and mortar fire.

Moscow said Russian forces had killed “more than 70 Ukrainian terrorists” and had destroyed several armed vehicles during the skirmishes, but AFP was unable to independently verify the claims. Russia said the remaining fighters had been driven back across the border.

The alleged perpetrators of the incursion gave interviews to Ukrainian media on Wednesday in which they hailed the “success” of the operation which they said highlighted the weakness of Russia’s defences.

“I want to prove to them that we can fight against tyrants and that (Russian President Vladimir) Putin’s power is not unlimited,” said Denis Kapustin of the Russian Volunteer Corps which claimed the sortie.

Kapustin, whose nickname is “White Rex”, is a well-known figure in hooligan and far-right circles in Russia.

While Ukraine has denied any responsibility for the incursion, Kapustin said Kyiv had “encouraged” it, without providing weapons or equipment.

Separately, Russia said Wednesday it had fended off a Ukrainian attack on one of its warships in Turkish waters with unmanned vessels, the latest attempted sabotage attack Moscow has blamed on Kyiv.

It added that the Russian vessel had been tasked with guarding pipeline infrastructure in Turkish economic waters.

The statement said that all three enemy boats have been destroyed and that the attack took place some 140 kilometres (90 miles) northeast of the Bosphorus.

Turkey did not immediately respond to Russia’s claims.

- Wagner’s ragtag forces -

Wagner chief Yevgeny Prigozhin, whose ragtag forces have been leading an assault for the eastern city of Bakhmut, said around 10,000 convicts that he had recruited to fight with Wagner were killed on the battlefield in Ukraine.

“I took 50,000 prisoners of which around 20 percent were killed,” Prigozhin said in a video interview published late Tuesday.

He said a similar percentage were killed among those who had signed a contract with Wagner, but did not give a precise figure.

Prigozhin, who has criticised the regular army and blamed Shoigu for Moscow’s massive losses, also said Russia’s border with Ukraine was not properly protected.

“Sabotage groups cross the Belgorod region absolutely calmly,” he said in the video interview.

Convicts are believed to have been used as cannon fodder in Ukraine, accounting for most of Wagner’s losses in the pro-Western country.

- ‘Cannot hide from this’ -

Last year, Prigozhin toured Russian prisons in a bid to convince inmates to fight with Wagner in Ukraine, in exchange for an amnesty upon their return should they survive.

In early May, the White House said that more than 20,000 Russian troops had died and another 80,000 were wounded in five months of fighting in eastern Ukraine, particularly in Bakhmut.

Prigozhin, whose influence has risen hugely during the more than year-long offensive, has scathingly criticised Russia’s top brass, accusing them of being responsible for huge losses.

“There are now tens of thousands of relatives of those who were killed. Probably there will be hundreds of thousands. We cannot hide from this,” he said.

Russia also announced on Wednesday that it will try five foreign men – three British citizens, a Swede and a Croatian – for fighting alongside Ukrainian troops.

The trial will take place in the southern city of Rostov-on-Don and the five men are believed to face trial in absentia.

The men have been accused of fighting alongside Ukrainian forces – including the Azov regiment, which battled against Russian forces during the siege of the southern port city of Mariupol.

The Britons have been identified as John Harding, Andrew Hill, and Dylan Healy. The Swede has been named as Mathias Gustafsson and the Croatian as Vjekoslav Prebeg.