The world narrowly avoided a radiation accident at the Zaporizhzhia nuclear station in Ukraine today after power was cut to the two remaining working reactors, President Volodymyr Zelensky said.
Ukraine's state nuclear company Energoatom said fires in the ash pits of a coal power station near the complex had disconnected the reactors from the power grid.
The company blamed Russian "invaders" for the disconnection.
Mr Zelensky, confirming earlier comments to Reuters by an energy official, said back-up diesel generators had immediately kicked in to ensure continuous power supply.
Electricity is used for cooling and safety systems at the nuclear plant.
"If the diesel generators had not turned on ... if our station staff had not reacted after the blackout, then we would have already been forced to overcome the consequences of a radiation accident," he said in an evening address.
"Russia has put Ukraine and all Europeans in a situation one step away from a radiation disaster."
He also called on the International Atomic Energy Agency and other world bodies need to act much faster to force Russian troops to leave the territory of the station, the largest of its kind in Europe.
"Every minute that Russian troops remain at the nuclear power station there is a risk of global radiation catastrophe," Mr Zelensky said in a video address.
Earlier, the UN nuclear watchdog said the Zaporizhzhia plant "at least twice lost connection to the power line during the day" but is "currently up again".
The International Atomic Energy Agency did not specify the cause of the outage but fires had earlier damaged overhead power lines, as shown in satellite images.
UN Secretary General Antonio Guterres has called for the area surrounding the reactor complex to be demilitarised.
This evening, the White House again called on Russia to agree to a demilitarised zone around the plant.
While the Ukrainian Energy Minister said that the IAEA could visit the facility "in the coming days".
Earlier, Energoatom - which runs the facility - said on Telegram, that Russian forces had "caused a complete disconnection of the (Zaporizhzhia facility) from the power grid - the first in the history of the plant."
It is Europe's largest nuclear power generator, and has been occupied by invading troops since the opening weeks of the war.
Moscow and Kyiv have traded blame over shelling around the complex in southern Ukraine.
Energoatom said the plant was disconnected from Ukraine's national supply system after a power line was twice disconnected by fires at ash pits in an adjacent thermal power plant.
The three other power lines "were earlier damaged during terrorist attacks" by Russian forces, the operator said.
As a result, the two of the plant's six reactors still functioning "were disconnected from the network".
Energoatom said "start-up operations are under way to connect one of the reactors to the network".
Kyiv officials have said they believe Moscow has seized the station in order to divert power to the Crimean peninsula annexed by Russia in 2014.
Energoatom could not be immediately reached for comment on whether the supply had been diverted.
It comes as Russia's defence ministry confirmed that it carried out an attack on a Ukrainian railway station that killed 25 people, including two children.