The plant "was fully disconnected from the power grid," Ukrenergo said in a statement on its Facebook page, adding that military operations meant "there is no possibility to restore the lines".

On February 24, Russia invaded Ukraine and seized the defunct plant, site of a 1986 disaster that killed hundreds and spread radioactive contamination west across Europe.

On Tuesday the UN atomic watchdog the International Atomic Energy Agency (IAEA) said that the site was no longer transmitting data and voiced concern for staff working under Russian guard.

The situation for the staff "was worsening", the IAEA said, citing the Ukrainian nuclear regulator.

The defunct plant sits inside an exclusion zone that houses decommissioned reactors as well as radioactive waste facilities.

More than 2,000 staff still work at the plant as it requires constant management to prevent another nuclear disaster.

IAEA Director General Rafael Grossi on Tuesday called on "on the forces in effective control of the site to urgently facilitate the safe rotation of personnel there."

He also repeated his offer to travel to Chernobyl or elsewhere to secure "the commitment to the safety and security" of Ukraine's power plants from all parties.

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