KYIV, Ukraine, June 6, 2023 (BSS/AFP) - Russia's claims that its military had inflicted massive casualties on advancing Ukrainian troops are "wild fantasies", the boss of Russian mercenary group Wagner said Tuesday.

Moscow's defence ministry said Monday it had thwarted a Ukrainian offensive, killing a total of "1,500 servicemen" and destroying more than 100 armoured vehicles.

"To destroy one and a half thousand people, it must be such a massacre, within one day, over 150 kilometres (90 miles), one hell of a massacre," Wagner chief Yevgeny Prigozhin said on Telegram in response to the claims.

"Therefore, I think that these are just some wild fantasies."

Prigozhin also said Monday that Kyiv's troops had made gains near the eastern Ukrainian city of Bakhmut, which Russia claimed to have seized control of last month.

Russian troops, he said, were "slowly" leaving the village of Berkhivka near Bakhmut, which he said was a "disgrace".

Prigozhin has been embroiled in a public spat with Russia's regular army and has accused Moscow's military leadership of not supplying enough ammunition, among other criticisms.

Ukrainian President Volodymyr Zelensky on Monday praised his troops for advances claimed near Bakhmut, the scene of the longest and one of the bloodiest battles of the war.

"Well done, warriors! We see how hysterically Russia reacts to any step we take there, all positions we take. The enemy knows that Ukraine will win," Zelensky said in a video message published on social media.

Ukraine says it has been preparing a major offensive to recapture territory lost to Russia, but that there would be no announcement about when it would start.