More than a dozen powerful explosions have been recorded near a huge Russian-occupied nuclear power plant in south Ukraine since Saturday evening.
The head of the UN nuclear watchdog, Rafael Grossi, made an urgent appeal for a stop to the fighting at the Zaporizhzhia plant, Europe's biggest.
"Whoever is behind this, it must stop immediately," he said. "You're playing with fire!"
The plant stands on the River Dnipro, on the front line in the war.
Russia's military accused Ukrainian forces on the other side of the river of shelling the area under its control. There was no immediate word from the Ukrainians who have previously suggested Russian forces shell the area themselves despite having their troops there.
The area around the plant, including the nearby Russian-occupied town of Enerhodar, had been under regular attack for months but there had been a period of calm before the new explosions this weekend, which continued into Sunday morning.
Monitors from Mr Grossi's organisation, the International Atomic Energy Agency (IAEA), witnessed some of the explosions from their windows.
Citing information provided by officials at the Russian-controlled plant, the IAEA team said there had been damage to some buildings, systems and equipment at the site, but nothing so far "critical for nuclear safety and security". There were no reports of casualties.
"The news from our team yesterday and this morning is extremely disturbing," Mr Grossi said. "Explosions occurred at the site of this major nuclear power plant, which is completely unacceptable."
He called once again for the two warring sides to agree and implement a nuclear safety and security zone around the plant as soon as possible.
"I'm not giving up until this zone has become a reality," he said. "As the ongoing apparent shelling demonstrates, it is needed more than ever."
Russian state media quoted an official from Russian nuclear power operator Rosenergoatom as saying 15 shells had been fired at the plant's facilities, landing near a dry nuclear waste storage facility and a building that houses fresh spent nuclear fuel, but no radioactive emissions had been detected.
The plant was overrun by Russian forces a few weeks after Moscow invaded Ukraine on 24 February.
Russia annexed the Zaporizhzhia region and other Ukrainian territory in September but has been pushed back on the battlefield in the south, notably in Kherson region, and the two armies face each other across the River Dnipro (known as the Dnepr in Russian).
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