Europe's largest atomic energy plant -- which is in a cold shutdown mode -- lost power on September 23, the longest of the 10 outages from the grid it has undergone since Russia seized the station following its invasion of Ukraine in February 2022.

The UN nuclear watchdog has also sought to quell concerns, saying late Tuesday that the plant faced no imminent danger, as long as backup electricity generators kept working.

"The situation at the Zaporizhzhia nuclear power plant is under control," the press service for the Russian operator of the plant said on social media, adding that it had enough fuel to keep backup electricity generators running.

It said radiation levels were normal and that it was in close contact with the International Atomic Energy Agency (IAEA), which has inspectors based at the plant.

Zelensky had said Tuesday the situation there was "critical" and that one of the backup diesel generators had "malfunctioned."

Moscow and Kyiv have repeatedly accused each other of risking a potentially devastating nuclear disaster by attacking the site and traded blame over the latest blackout.

The plant's six reactors, which before the war produced around a fifth of Ukraine's electricity, were shut down after Moscow took over.

But the site needs power to maintain the cooling and other safety systems that prevent the reactors from melting down and releasing radiation into the atmosphere.

- 'Last line of defence' -

The IAEA said it had been informed by the Russian-installed operators that they had fuel reserves for 10 more days, "with regular offsite supplies maintaining this level".

"While the plant is currently coping thanks to its emergency diesel generators -- the last line of defence -- and there is no immediate danger as long as they keep working, it is clearly not a sustainable situation in terms of nuclear safety," IAEA head Rafael Grossi said in a statement late Tuesday.

"Neither side would benefit from a nuclear accident... I'm in constant contact with the two sides with the aim to enable the plant's swift re-connection to the electricity grid," he added.

Since the start of the war, Zaporizhzhia has seen multiple safety threats, including frequent nearby shelling, repeated power cuts and staff shortages.

Located near the city of Energodar, the site sits on the Dnieper river, the de facto front line in southern Ukraine.