"Shelling has caused a complete blackout in Energodar (town) and compromised the safe operation of the nearby Zaporizhzhia (plant)," International Atomic Energy Agency (IAEA) chief Rafael Grossi tweeted, calling it a "dramatic development".

"This is completely unacceptable. It cannot stand," he said, calling for "the immediate cessation of all shelling in the entire area".

"Only this will ensure the security of operating staff and allow the durable restoration of power to Energodar and to the power plant," he added.

The Zaporizhzhia plant is Europe's largest nuclear power facility.

It was occupied by Russian troops in March and has been shelled in recent weeks, with Ukraine and Russia blaming each other for the attacks, raising fears of a nuclear disaster.

Grossi said he learned of the shelling on Friday from IAEA staff on the site.

The shelling of the switchyard at Energodar's thermal power plant has led to a "complete power black-out" in the city, where the Zaporizhzhia nuclear power plant is located, he said.

"No running water, no power, no sewage," he said.

- 'Significant risk' -

Plant operators and others living in Energodar faced "increasingly dire circumstances", with a "significant risk" there would no longer be enough essential staff at the plant.

And there was "little likelihood of re-establishing reliable offsite power" given the increased and continued shelling, he added.

"As a result, the IAEA understands that the operator, having no longer confidence in the restoration of offsite power, is considering shutting down the only remaining operating reactor," he said.

The plant in recent days has relied on this reactor for the power it needs for cooling and other safety functions.

"The entire power plant would then be fully reliant on emergency diesel generators for ensuring vital nuclear safety and security functions," Grossi warned.

The IAEA called Tuesday for a security zone to be set up around the Zaporizhzhia nuclear power plant, saying the current situation was "untenable".

The IAEA sent a 14-person team to the site last week. Two members are remaining there on a permanent basis to ensure the facility's safety.

Kyiv on Wednesday called for an international mission to be set up at the plant and for the population to evacuate the area amid fears of a nuclear disaster.