Ukraine's president and the UN nuclear agency head are sounding the alarm about increased safety risks at the Russia-occupied Zaporizhzhia Nuclear Power Plant in southern Ukraine, which lost its external power supply more than a week ago as the war raged around it.

Emergency diesel generators are providing power for crucial cooling systems for the facility's six shutdown reactors and spent fuel, and there is no immediate danger to Europe's biggest nuclear plant, according to International Atomic Energy Agency Director-General Rafael Mariano Grossi.

But "it is clearly not a sustainable situation in terms of nuclear safety," he said.

The backup generators have never needed to run for so long, according to Ukrainian President Volodymyr Zelenskyy.

"The generators and the plant were not designed for this," Zelenskyy said late Tuesday, describing the situation as "critical."  

Zaporizhzhia is one of the 10 biggest nuclear plants in the world, and its fate amid the fighting has caused fears of a potential nuclear catastrophe. Russian forces seized it days after the full-scale invasion of neighbouring Ukraine began on February 24, 2022.

Grossi said that Zaporizhzhia's emergency generators were coping with the extra strain so far.

"The current status of the reactor units and spent fuel is stable as long as the emergency diesel generators are able to provide sufficient power to maintain essential safety-related functions and cooling," he said in a statement late Tuesday.

An IAEA team at the plant reported that it has fuel reserves ensuring the generators can operate for more than 10 days, with regular off-site supplies maintaining this level.