Russian forces have disconnected Chernobyl nuclear power plant from the national grid igniting fear at the site, according to Ukraine.
National nuclear energy company Ukrenergo said the plant, now captured by Russian forces, is in total blackout with no power supply.
The lack of power at Chernobyl risks is also sending radioactive substances into the air.
Ukrenergo added: "Because of the military actions of the Russian occupiers, the nuclear power plant in Chernobyl was fully disconnected from the power grid.
"The nuclear station has no power supply.
"The military actions are in progress, so there is no possibility to restore the lines. Slavutich city is also out of power supply."
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Ukraine's state-run nuclear company Energoatom said radioactive substances could be released from the plant because it cannot cool spent nuclear fuel after its power connection was severed.
Work to repair the connection and restore power to the plant has been impossible due to ongoing fighting.
Energy Minister Herman Halushchenko said today Ukrainian authorities do not know what the radiation levels are.
He added they have not heard about what is happening there since it was seized by Russian troops.
Mr Halushchenko said Ukraine has no control over what was happening at the occupied Zaporizhzhia nuclear power plant, where it said 400 Russian troops were stationed.
Systems monitoring nuclear material at the radioactive waste facilities at Chernobyl in Ukraine, which were taken over by Russian forces last month.
Since that time the site has stopped transmitting data to the UN nuclear watchdog.
In a statement, the International Atomic Energy Agency said: "The Director General...indicated that remote data transmission from safeguards monitoring systems installed at the Chornobyl NPP had been lost."
Safeguards refer to the field of IAEA work aimed at keeping track of nuclear material.
Ukraine's nuclear operator fears the Chernobyl blackout may cause a radiation leak that could spread across Europe.
The State Service of Special Communications and Information Protection of Ukraine tweeted: "Kyiv high-voltage line is currently disconnected due to the damage caused by the occupiers.
"As a result, the Chernobyl station and all nuclear facilities in the Exclusion Zone are without electricity.
"About 20,000 spent fuel assemblies are stored in the spent nuclear fuel storage facility-1.
"They need constant cooling. Which is possible only if there is electricity.
"If it is not there, the pumps will not cool. As a result, the temperature in the holding pools will increase.
"After that evaporation will occur, that will lead to nuclear discharge.
"The wind can transfer the radioactive cloud to other regions of Ukraine, Belarus, Russia and Europe.
"In addition, there is no ventilation inside the facility.
"All personnel there will receive a dangerous dose of radiation. The fire extinguishing system also does not work, and this is a huge risk of fire caused by shelling.
"The fight still goes on making it impossible to carry out repairs and restore power."
Minister of Foreign Affairs of Ukraine Dmytro Kuleba spoke about the ongoing crisis today, warning "Putin's barbaric war puts entire Europe in danger."
He tweeted: "The only electrical grid supplying the Chornobyl NPP and all its nuclear facilities occupied by Russian army is damaged. CNPP lost all electric supply.
"I call on the international community to urgently demand Russia to cease fire and allow repair units to restore power supply.
"Reserve diesel generators have a 48-hour capacity to power the Chornobyl NPP.
"After that, cooling systems of the storage facility for spent nuclear fuel will stop, making radiation leaks imminent.
"Putin’s barbaric war puts entire Europe in danger. He must stop it immediately!"
