"The resumption of shelling, hitting the plant's sole source of external power, is tremendously irresponsible," International Atomic Energy Agency (IAEA) director general Rafael Grossi said.

Although the six reactors are in cold shutdown, they require electricity for vital nuclear safety and security functions, including cooling.

Grossi said he would soon travel to Russia before returning to Ukraine "to agree on a nuclear safety and security protection zone around the plant."

Russian President Vladimir Putin ordered his government this week to take over operations at Europe's largest nuclear power station in southern Ukraine, where fears of an atomic accident remain high.

At the site, the diesel generators each have enough fuel to run for at least 10 days, but engineers have begun work to repair the power line.

The plant's Ukrainian operator Energoatom said Russian shelling had "damaged and disconnected" the last power link.

Grossi was in Kyiv on Thursday to discuss setting up a protection zone around the site which has regularly come under fire with Ukraine and Russia blaming each other.

He stressed his main aim was "to avoid a nuclear accident at the plant, which remains a very, very clear possibility".

Electricity has been cut to the site several times since August.

Early in Russia's invasion of Ukraine, there was fighting around Chernobyl in the north, where a 1986 explosion left swathes of the surrounding territory contaminated by fallout.