Kyiv on Monday accused Russia of attacking Ukraine’s second largest nuclear power plant in the war-torn country’s south, the latest outbreak of fighting over nuclear sites that has raised fears of a radiation emergency.

The Kremlin, meanwhile, has dismissed outright claims that its forces were responsible for mass killings in recently conquered areas of eastern Ukraine, saying Ukraine’s claims that it had uncovered mass graves were fabricated.

Ukraine’s atomic 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 meters (985 feet) from its reactors.

The strike damaged more than 100 windows of the power plant’s building, but the reactors were operating normally, according to the agency, which released photos of broken glass around blown frames.

It also released images of a crater said to be two meters deep, from which the rocket landed.

“Fortunately, none of the power plant employees were injured,” Energoatom said.

Attacks on nuclear facilities in Ukraine have prompted calls from Ukraine and its western allies to demilitarize areas around the facilities.

Europe’s largest nuclear facility – the Zaporizhzhia nuclear power plant on Russian-held territory in Ukraine – has become a focus of concern after allegations of attacks there were recited.

At the start of the Russian invasion of Ukraine – which began in late February – there was fighting around Chernobyl in the north, where a 1986 explosion contaminated much of the surrounding territory.

President Volodymyr blamed Russia for Monday’s attack in the southern Mykolayiv region, which he said led to a brief power outage at the plant.

“Russia endangers the whole world. We have to stop it before it’s too late,” Zelenskyy said on Telegram.

The Zaporizhia plant was seized by Russian troops in March and shelling of the plant has prompted intervention by Western leaders. A monitoring team from the UN nuclear agency was stationed there in early September.

French President Emmanuel Macron this month called on Vladimir Putin to withdraw Russian heavy weapons from the region, while the Russian president warned of the possible “catastrophic” consequences of fighting there.

The Mykolaiv region of southern Ukraine, where the Pivdennoukrainsk plant is located, is the close front line of a southern Ukrainian counteroffensive against Russian forces.

Kiev’s forces have slowly but steadily reclaimed territory in the southern Kherson region adjacent to Mykolaiv with the aim of capturing the strategically important hub, also known as Kherson.

Ukraine’s advances have been faster in the north, where Kyiv’s forces recaptured almost the entire Kharkiv region in a lightning strike this month.

These achievements have dealt a severe blow to Moscow’s ambitions to seize and hold Donbass, an industrial region in eastern Ukraine 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,” Zelenskyy said in an address to the nation on Sunday evening.

“But this is not a break. This is preparation… all Ukraine must be free,” he said.

The recapture of cities like Kupyansk and Izyum, which were key hubs on Russia’s supply routes, means Moscow will have greater difficulty supplying frontline positions elsewhere in eastern Ukraine.

They have also come up with fresh allegations of atrocities committed by Russian forces during their months-long occupation of towns and settlements in the Kharkiv region, particularly after the discovery of mass graves.

The Kremlin on Monday denied that Russian forces were responsible for mass killings and dismissed the claims as fabricated.

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

However, civilians in cities retaken from Ukraine have told of Russia’s brutal occupation.

In Kupyansk, a city in Kharkiv, Mykhailo Chindey said he was tortured by Russian troops who suspected him of providing coordinates to Ukrainian forces.

“One person held my hand and another hit my arm with a metal stick. They beat me for two hours almost every day,” he told AFP.

“At one point I lost consciousness. I lost a lot of blood. They hit my heels, my back, my legs and my kidneys,” he said.

Meanwhile, Russian forces continued to shell Ukrainian-held towns near the front lines.

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

In the Donetsk region, five civilians were killed and another 18 injured in a Russian shelling, Kyiv said.