(Reuters) - Russian forces seized the largest nuclear power plant in Europe after a building at the complex was set ablaze during intense fighting with Ukrainian defenders, Ukrainian authorities said on Friday.

Fears of a potential nuclear disaster at the Zaporizhzhia plant had spread alarm across world capitals, before authorities said the fire in a building identified as a training centre, had been extinguished.

U.S. Energy Secretary Jennifer Granholm said there was no indication of elevated radiation levels at the plant, which provides more than a fifth of total electricity generated in Ukraine.

The Ukrainian regional authority confirmed in a Facebook post that Russian forces had captured the plant and said personnel were monitoring the condition of power units to ensure they could operate safely.

Earlier, a video feed from the plant verified by Reuters showed shelling and smoke rising near a five-storey building at the plant compound.

The footage shot at night showed one building aflame, and a volley of incoming shells, before a large candescent ball 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," Ukrainian leader Volodymyr Zelenskiy said in a video address.

Zelenskiy said Russian tanks had shot at the nuclear reactor plants, though there was no evidence cited that they had been hit.

The mayor of the nearby town of Energodar about 550 km (342 miles) southeast 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 and more than 1 million refugees have fled Ukraine since Russian President Vladimir Putin last Thursday launched the biggest attack on a European state since World War Two.

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 over-reaction and the war prolongs," said Vasu Menon, executive director of investment strategy at OCBC Bank.

Russia had already captured the defunct Chernobyl plant north of Kyiv, which spewed radioactive waste over much of Europe when it melted down in 1986. The Zaporizhzhia plant is a different and safer type, analysts said.