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Russian forces have seized the Zaporizhzhia nuclear power plant in Ukraine's south-east, a local authority said on Friday.
The plant is Europe's biggest, and provides more than a fifth of the total electricity generated in Ukraine.
“Operational personnel are monitoring the condition of power units,” the Zaporizhzhia regional military administration said on social media. It said it was trying to ensure the plant was operating in line with safety requirements.
Ukraine has said Russian forces attacked the plant in the early hours of Friday, setting an adjacent five-story training facility on fire.
Earlier, Ukraine’s state emergency service said the fire had been extinguished. The local authority also said the blaze had been put out. It added that there was damage to a compartment of reactor No1 but it did not affect the safety of the power unit.
US Energy Secretary Jennifer Granholm said there was no indication of elevated radiation levels at the Zaporizhzhia plant.
Earlier, a video feed from the plant verified by Reuters showed shelling and smoke rising near the five-storey building at the compound.
The footage shot at night showed one building aflame, and a volley of incoming shells, before a large fireball lit up the sky, exploding beside a car park and sending smoke billowing across the compound.
“Europeans, please wake up. Tell your politicians — Russian troops are shooting at a nuclear power plant in Ukraine,” Ukraine's President Volodymyr Zelenskyy said in a video address.
He accused Mr Putin of “nuclear terror” and said that attack risked sparking a disaster “like six Chernobyls”.
“This night could have been the end of history for Ukraine and Europe,” he warned on Friday morning.
British Prime Minister Boris Johnson said the Russians’ shelling of the plant could “directly threaten the safety of all of Europe”.
The mayor of the nearby town of Energodar, about 550 kilometres south-east of Kyiv, said fierce fighting and “continuous enemy shelling” had caused casualties in the area, without providing details.
Thousands of people are believed to have been killed or wounded while more than a million refugees have fled Ukraine since last Thursday when Russian President Vladimir Putin launched the biggest attack on a European state since the Second World War.
Early reports of the incident at the power plant sent financial markets in Asia spiralling, with stocks tumbling and oil prices surging further.
“Markets are worried about nuclear fallout. The risk is that there is a miscalculation or overreaction and the war prolongs,” said Vasu Menon, executive director of investment strategy at OCBC Bank.
Russia has already captured the defunct Chernobyl plant, about 100km north of Kyiv, which spewed radioactive waste over much of Europe during a meltdown in 1986. The Zaporizhzhia plant is a different and safer type, some analysts said.
US President Joe Biden and UK Prime Minister Boris Johnson both spoke with Mr Zelenskyy to receive an update on the situation at the plant.
“President Biden joined President Zelenskyy in urging Russia to cease its military activities in the area and allow firefighters and emergency responders to access the site,” the White House said.
Nato Secretary General Jens Stoltenberg said the military alliance will not impose a no-fly zone over Ukraine as such a move could result in widespread war in Europe.
Speaking on Friday after leading a meeting of Nato foreign ministers, Mr Stoltenberg said: “We are not going to move into Ukraine, neither on the ground, nor in the Ukrainian airspace.”
Mr Zelenskyy has appealed to the West to enforce a no-fly zone over his country.
“The only way to implement a no-fly zone is to send Nato fighter planes into Ukrainian airspace and then impose that no-fly zone by shooting down Russian planes,” Mr Stoltenberg said.
“We understand the desperation, but we also believe that if we did that, we would end up with something that could end in a full-fledged war in Europe.”