A view shows the Zaporizhzhia Nuclear Power Plant
A view shows the Zaporizhzhia Nuclear Power Plant in the course of Russia-Ukraine conflict outside Enerhodar in the Zaporizhzhia region, Russian-controlled Ukraine, March 29, 2023. Reuters

The Russian-occupied Zaporizhzhia nuclear power plant in southern Ukraine has been cut off from its external power supply and is relying on emergency generators to cool nuclear fuel and prevent a disaster.

Each side blamed the other for the power outage on Monday. A Russia-installed local official said Ukraine had disconnected a power line and Ukrainian state nuclear energy company Energoatom said the outage was caused by Russian shelling.

Confirming the outage, the head of the United Nations nuclear energy watchdog said the "nuclear safety situation at the plant (is) extremely vulnerable."

"We must agree to protect (the) plant now; this situation cannot continue," Rafael Grossi, head of the International Atomic Energy Agency (IAEA), wrote on Twitter.

Even though the six reactors are shut down at the plant, they still need a constant supply of electricity to keep the nuclear fuel inside cool and prevent a possible meltdown.

Energoatom said the back-up diesel generators had enough fuel for about 10 days.

"The countdown has begun," it said in a statement posted online.

The plant, which lies in an area of Russian-occupied southern Ukraine near front lines along the Dnipro River, is Europe's biggest nuclear power station and the area has been repeatedly hit by shelling.

Energoatom said it was the seventh time power had been cut to the plant since Russia's full-scale invasion of Ukraine in February 2022. On all the previous occasions, the external power has been restored after the emergency generators kicked in.

Before Russia's invasion, the plant provided about a fifth of Ukraine's electricity needs.

The Ukrainian energy ministry said nearly 250,000 consumers had lost power in the Zaporizhzhia region as a result of damage to equipment at substations in the latest Russian shelling, but that power had already been restored to most of them.