Although the plant was now said to be safe and the fire out, officials remained worried about the precarious circumstances, with Ukrainian staff operating under Russian controls.

Russian forces seized Europe's biggest nuclear power plant on Friday in heavy fighting in south-eastern Ukraine, triggering global alarm, but a huge blaze in a training building has been extinguished and officials said the facility was now safe.

Combat raged elsewhere in Ukraine as Russian forces surrounded several cities in the second week of the assault launched by Russian President Vladimir Putin.

A presidential advisor said an advance had been halted on the southern city of Mykolayiv after local authorities said Russian troops had entered it. If captured, the city of 500,000 people would be the biggest yet to fall.

The capital Kyiv, in the path of a huge Russian armoured column that has been stalled on a road for days, came under renewed attack, with air raid sirens blaring in the morning and explosions audible from the city centre. The fire at a training building at the Zaporizhzhia nuclear power plant sent global stock markets plunging.

Although the plant was now said to be safe and the fire out, officials remained worried about the precarious circumstances, with Ukrainian staff operating under Russian control in battlefield conditions beyond the reach of administrators.

International Atomic Energy Agency chief Raphael Grossi described the situation as "normal operations, but in fact there is nothing normal about this".

He paid homage to the plant's Ukrainian staff: "to their bravery, to their courage, to their resilience because they are doing this in very difficult circumstances."

Grossi said the plant was undamaged from what he believed was a Russian projectile. Only one reactor was working, at around 60% of capacity. He was trying to contact Russian and Ukrainian officials to sort out political responsibility.

An official at Energoatom, the Ukrainian state nuclear plant operator, said there was no further fighting and radiation was normal, but his organisation no longer had contact with the plant's management or control over potentially dangerous nuclear material.

"Personnel are on their working places providing normal operation of the station," the official said.

Russia's defence ministry also said the plant was working normally. It blamed the fire on a "monstrous attack" by Ukrainian saboteurs and said its forces were in control.

A video from the plant verified by Reuters showed one building aflame and a volley of incoming shells before a large incandescent ball lit up the sky, exploding beside a car park and sending smoke billowing across the compound.

"Europeans Wake Up"

Russia's grip on a plant that provides more than a fifth of Ukraine's electricity was a big development after eight days of war in which other Russian advances have been stalled by fierce resistance.

"Europeans, please wake up. Tell your politicians – Russian troops are shooting at a nuclear power plant in Ukraine," Ukrainian President Volodymyr Zelenskiy said in a video address. In another address, he called on Russians to protest.

Thousands of people are believed to have been killed or wounded and more than 1 million refugees have fled Ukraine since Feb. 24, when Putin ordered the biggest attack on a European state since World War Two.

Russian forces advancing from three directions have besieged cities, pounding them with artillery and air strikes. Moscow says its aim is to disarm its neighbour and capture leaders it calls neo-Nazis and a threat to its own security.

Ukraine and its Western allies call that a baseless pretext for a war to conquer a country of 44 million people.