Power has been cut to the Chernobyl power plant, the site of the world's worst nuclear disaster, Ukraine said Wednesday, but the UN's atomic watchdog said there was "no critical impact on safety".

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

It said that there was also no power to the site's security systems.

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.

The UN atomic watchdog, the International Atomic Energy Agency (IAEA) said in a tweet that while the development "violates (a) key safety pillar", in this case it saw "no critical impact on safety".

On Tuesday the IAEA had 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. 

Ukrainian Foreign Minister Dmytro Kuleba tweeted on Wednesday that "reserve diesel generators have a 48-hour capacity to power" the plant but added that "after that, cooling systems of the storage facility for spent nuclear fuel will stop".

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 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.