Kyiv accused Russia on Monday of attacking Ukraine's second-largest nuclear plant in the south of the war-scarred country, the latest burst of fighting around atomic facilities that has raised fears of a radiation emergency.

The Kremlin meanwhile dismissed outright claims that their forces had been responsible for mass killings in recently captured areas of east Ukraine and said Ukraine's claims it had discovered mass graves were made up.

Ukraine's nuclear energy agency, Enargoatom, said the Russian army "carried out a missile attack" on the industrial site of the Pivdennoukrainsk nuclear power plant, with a "powerful explosion" just 300 metres (985 feet) from its reactors.

The strike damaged more than 100 windows of the power station's building, but the reactors were operating normally, according to the agency, which published photos of glass shattered around blown-out frames.

It also released images of what it said was a two-metre-deep crater from where the missile landed.

"Fortunately, no one among the power plant's staff was hurt," Energoatom said.

Attacks around nuclear facilities in Ukraine have spurred calls from Ukraine and its Western allies to de-militarise areas around the facilities.

Europe's largest atomic facility -- the Zaporizhzhia nuclear plant in Russian-held territory in Ukraine -- has become a hot spot for concerns after tit-for-tat claims of attacks there.

'We have to stop' Russia 

Early in Russia's invasion in of Ukraine -- launched in late February -- there was fighting around Chernobyl in the the north, where an explosion in 1986 left swathes of the surrounding territory contaminated.

President Volodymyr blamed Russia for the attack in the southern Mykolaiv region on Monday, which he said resulted in a short power outage at the facility.

"Russia endangers the whole world. We have to stop it before it's too late," Zelensky said on Telegram.

The Zaporizhzhia plant was seized by Russian troops in March and shelling around the facility has spurred interventions from Western leaders. A monitoring team of the UN's atomic agency deployed there in early September.

French President Emmanuel Macron this month urged Vladimir Putin to withdraw Russian heavy weapons from the region, while the Russian president cautioned against potential "catastrophic" consequences of fighting there.

The Mykolaiv region in southern Ukraine, where the Pivdennoukrainsk plant is located, is the near frontline of a Ukrainian counter-offensive in the south against Russian forces.

Kyiv's forces have slowly but steadily been clawing back territory in the southern Kherson region, next to Mykolaiv, with the aim of capturing the strategically important hub, also called Kherson.

Ukraine's progress has been faster in the north, where a lightning grab this month has seen Kyiv's forces reconquer nearly the entire Kharkiv region.

Those gains have delt a serious blow to Moscow's ambitions of capturing and holding Donbas, a industrial region of eastern Ukraine that has been partially controlled by Kremlin-backed rebels since 2014.

"It may seem to some of you that after a series of victories we have a certain lull," Zelensky said in an address to the nation on Sunday evening.

"But this is not a lull. This is preparation ... the whole Ukraine must be free," he said.

Mass grave 'lies'

The recapture of cities like Kupiansk and Izyum, which were key hub on Russian resupply routes mean Moscow will have greater difficulty supplying frontline positions elsewhere in east Ukraine.

They have also brought fresh claims of atrocities committed by Russian troops during their months-long hold of Kharkiv-region towns and settlements, particularly after the discovery of mass burial sites.

The Kremlin on Monday denied Russian forces were responsible for mass killings, dismissing the claims as fabricated.

"These are lies," Kremlin spokesman Dmitry Peskov told reporters Monday. Moscow "will stand up for the truth in this story."

Civilians in towns and cities recaptured by Ukraine, however, have recounted Russia's brutal occupation.

In Kupiansk, a town in Kharkiv, Mykhailo Chindey, said he had been tortured by Russian troops who suspected him of supplying coordinates to Ukrainian forces.

"One person was holding my hand and another one was beating my arm with a metal stick. They were beating me up two hours almost every day," he told AFP.

"I lost consciousness at some point. I lost a lot of blood. They hit my heels, back, legs and kidneys," he said.

Russian forces have meanwhile continued shelling Ukrainian-held towns near the frontlines.

The Ukraine presidency said that Russian forces remaining in the Kharkiv region had fired on a civilian car on Sunday, killing two women.

In the Donetsk region, Russia shelling killed five civilians and injured another 18 people, Kyiv said.

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