The U.N.’s nuclear watchdog calls for protection of the Zaporizhzhia plant.

As Russia targets Ukraine’s energy infrastructure, the head of the United Nations nuclear watchdog has emphasized the need to safeguard the Russian-occupied Zaporizhzhia nuclear plant, Europe’s largest, to prevent a nuclear disaster.

“Until we have this plant protected, the possibility of the nuclear catastrophe is there,” Rafael Mariano Grossi, the director general of the International Atomic Energy Agency, said in an interview with CBS News on that aired on Sunday.

Russia’s invasion has endangered Ukraine’s nuclear power facilities, which rely on external power to cool their reactors. At Zaporizhzhia, those power lines have been repeatedly damaged in the fighting, forcing operators of the plant to turn occasionally to diesel generators to run the cooling systems. Mr. Grossi said that resorting to diesel generators was an unsustainable practice given how frequently external power is lost.

“You don’t want the biggest nuclear power plant in Europe, one of the biggest in the world, to be cooled with — basically an emergency system which is dependent on fuel,” he said. “Because when your generators are out of whatever you put in it to make them work, then what happens? Then you have a meltdown. Then you have a big radiological nuclear emergency or an accident, and this is what we are trying to prevent.”

In October, Mr. Grossi spoke with both President Vladimir V. Putin of Russia and President Volodymyr Zelensky of Ukraine, hoping to get both sides to agree to establishing a demilitarized zone around Zaporizhzhia. He has been calling for similar measures since leading a team of inspectors to the facility in August. No agreement has been reached.

Last week, in one of the biggest and broadest attacks since the start of the war in February, Russia in one day fired more than 100 missiles at Ukraine, most of which were shot down by Ukrainian forces. The rest struck key energy infrastructure, plunging nearly a quarter of the country into darkness. Ukraine has warned that Russia will not relent in its attacks on infrastructure, making it harder for Ukrainians to stay warm as winter approaches.

Before the war, nuclear plants supplied a large share of Ukraine’s power. In the past month Russia expanded its attacks on Ukrainian nuclear facilities, targeting the Khmelnytskyi plant in western Ukraine and forcing it to switch to diesel generators for several hours. A second nuclear power plant in the nearby province of Rivne was forced to reduce the energy it produced after power lines were damaged.

At the Zaporizhzhia power plant, the reactors have been shut down since September as a safety measure. While a direct military strike on one of the reactor cores could still trigger an accident, the risk is greatly reduced if the plant is not operating.

When a dozen shells exploded near the Zaporizhzhia plant on Sunday, Russian and Ukrainian nuclear energy officials blamed each other’s military forces for the attack.

Mr. Grossi had inspected the Zaporizhzhia plant in August and observed large holes on the rooftop of a storage facility for nuclear fuel. The electrical switchyard at the plant had also been shelled, he said, suggesting that Russia had been trying to cut off the plant’s access to electricity. Workers at the facility have also been held hostage by Russian soldiers.

Mr. Grossi said that he hoped that both sides would stop attacks that threaten the plant.

“A demand for protection of the plant is very important,” Mr. Grossi said. “You don’t shell a nuclear power plant. You don’t storm a nuclear power plant.”