A fire erupted in the early hours of the morning at Europe's largest nuclear power plant in Ukraine as Russian troops shelled it from all sides.
The Zaporizhzhia nuclear power station in southeast Ukraine has reportedly been seized by Russian military forces after a fierce battle overnight.
It has sparked fears that if it explodes it would be "ten times worse than Chernobyl ".
However, experts have assured that there was no release of radioactive material.
Ukrainian authorities later confirmed that Russian forces seized the Zaporizhzhia nuclear plant.
"Operational personnel are monitoring the condition of power units," a local authority said on social media, quoted by Reuters news agency.
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Station personnel continue to work and monitor the state of the power units, it said.
This morning's attack marks a severe escalation in the Russian invasion of Ukraine.
We've answered the pressing questions you may have.
What happened?
Ukrainian authorities said a fire broke out in a training building outside the plant in the early hours of March 4 after Russian forces shelled from all sides.
The alarm was raised by an employee of the plant, who shared it on Telegram, saying there was "a real threat of nuclear danger at the largest power plant in Europe".
By 2.30am local time, Ukraine's foreign minister confirmed the reports and tweeted that the Russian army was "firing from all sides upon Zaporizhzhia NPP, the largest nuclear power plant in Europe. Fire has already broke out.”
Shortly after, the Ukrainian State Emergency Service reported that radiation at the plant was “within normal limits” and the fire conditions at the plant were “normal”. It added the fire was in a building outside the power plant.
By 2.26am, they reported the third power unit at the plant was disconnected, leaving just one of the four units operating.
The fire has since been extinguished.
Is there a radiation threat?
Ukrainian authorities this morning that the facility was secured and “nuclear safety is now guaranteed”.
Earlier, the International Atomic Energy Agency said the Ukraine regulator confirmed there was “no change reported in radiation levels at the Zaporizhzhia nuclear power plant site”.
This morning, IAEA Director General Rafael Grossi says there was no release of radioactive material at the nuclear power plant during the fire.
He added that two security personnel were injured.
What do experts think?
Ukraine is heavily dependent on nuclear energy and it has 15 reactors at four different plants generating about 54 per cent of the country's electricity, according to the World Nuclear Association.
The Zaporizhzhia nuclear power station generates around 20 per cent of all electricity in Ukraine.
Andrew Wilson, Professor of Ukrainian Studies at University College London, told the Mirror this attack could be an attempt to cut off the population from their services.
Prof Wilson said: "This is one of many nuclear power plants in Ukraine in the of the Russian advance.
"They are not making much progress from the East and their current strategy appears to be to surround main cities and cut off basic services.
"This is Russia's Modus Operandi they target the civilian population to make them suffer and make them lose faith in their democratic leaders because they are no longer getting basic services.
"Russia's hope is that Ukraine will surrender which I don't think it will.
"So whether the shelling was part of the strategy - one hopes not and that it was a rogue local unit because these things are built for many things like earthquakes but they are not built for missiles or shelling."
"An optimistic reading is that this is a brutal strategy to cut the population off from basic services rather than blow the whole place up.
"The fact that Ukraine is a neighbour of Russia should be a deterrent because if the wind blows in the wrong direction it will hit them too."
Other experts have said that attacking a nuclear plant was unprecedented.
Dr Graham Allison, a nuclear security expert at Harvard University, told the BBC the "worst case" would be if a fire at the plant caused a meltdown and caused a release of radioactivity that contaminated the surrounding area for years.
He also said it was more likely Russian forces were trying to "close down the supply of electricity to the surrounding area", rather than attack the plant.
Why is the power plant important?
The Zaporizhzhia nuclear power plant is the largest nuclear power plant in Europe and the ninth-largest in the world.
It was built between 1984 and 1995 and has six reactors.
It has six reactors, each generating 950MW, and a total output of 5,700MW, enough energy for roughly four million homes.
In normal times it produces one-fifth of Ukraine’s electricity and almost half the energy generated by the country’s nuclear power facilities.
It is about 200km from the contested Donbas region and 550km south-east of Kyiv.
What has been said about it?
Ukraine President Volodymyr Zelensky has said that everyone in Europe needs to be worried about the threat from Putin and a nuclear disaster.
In his message to the West he said: "For the first time in our history, the history of mankind, the terrorist state has resorted to nuclear terror.
"Russian protagonists have threatened to cover the world with nuclear ashes, now it is not a threat, now it is a reality. We must stop the Russian military immediately.
"If there is an explosion, it is the end of everything, the end of Europe. It is the evacuation of Europe. Only immediate European action can stop Russian troops. Prevent the death of Europe from the nuclear disaster."
Zelensky said that an explosion at the plant would be a catastrophe and much worse than that which happened at Chernobyl.
"It is a global catastrophe. Hundreds of thousands of people fought with its consequences, tens of thousands of people were evacuated," the president said of the Russian invasion.
He said: " Russia wants to do it again and is doing it again, six times more. Europeans, wake up, please.
"Tell your politicians Russian troops are shelling the nuclear power plant in Ukraine, Zaporizhzhya NPP, in Energodar. There are six power units, six! One power unit exploded in Chernobyl."
Meanwhile, Prime Minister Boris Johnson spoke with Zelensky after news broke about the fire at Zaporizhzhia.
A Downing Street spokeswoman called the situation “gravely concerning”, adding: “Both leaders agreed that Russia must immediately cease its attack on the power station and allow unfettered access for emergency services to the plant.
“The Prime Minister said the reckless actions of President Putin could now directly threaten the safety of all of Europe. He said the UK would do everything it could to ensure the situation did not deteriorate further.
“The Prime Minister said he would be seeking an emergency UN Security Council meeting in the coming hours, and that the UK would raise this issue immediately with Russia and close partners.
“Both leaders agreed a ceasefire was crucial.”
