The head of the International Committee of the Red Cross called on Thursday for all fighting near the Russian-controlled Zaporizhzhia power plant to stop, warning that little could be done to respond in the event of a major accident there.

"In the event of a nuclear leak, it will be difficult if not impossible to provide humanitarian assistance ... and this is why fighting should stop," Robert Mardini told a news conference during a visit to Ukraine.

"The scenario could be a massive incident, and ... there is very little anyone can do to mitigate the dire consequences of this," he said, when asked about contingency planning for an accident at Europe's largest nuclear plant.

Its Ukrainian operator said on Thursday it had shut down one of two operational reactors due to Russian shelling. It said the other was continuing to provide power for the plant's own needs.

Russia's defence ministry said it was using helicopters to destroy a Ukrainian "sabotage group" that it said had landed near the plant, Interfax news agency reported.

Conditions at Zaporizhzhia have been deteriorating for weeks, with Moscow and Kyiv trading blame for shelling in the vicinity and fuelling fears of a radiation disaster.

Mardini welcomed the planned visit by the United Nations' nuclear watchdog to inspect the damage, warning an accident could have catastrophic and long-lasting consequences for millions of people and the environment.

"It is therefore time to stop playing with fire and instead take concrete measures to protect this facility, and others like it, from military operations," he said. "The slightest miscalculation could trigger devastation that we will regret for decades."

Mardini told Reuters that the Red Cross was not stockpiling or distributing iodine tablets to people near the plant to avoid sowing panic.

"We don't want to give a signal that this is inevitable," he said. "This is totally self-inflicted risk that should be totally stopped."