The head of the world's nuclear watchdog said Wednesday external power to the Zaporizhzhia nuclear power plant in southeastern Ukraine has been restored after being cut when a substation suffered damage from Russian attacks.

International Atomic Energy Agency Director General Rafael Grossi tweeted that the outage -- the second in five days -- highlighted "how precarious the situation is." He said a protection zone should be set up around the Russian-occupied plant as soon as possible.

Ukraine's state nuclear power utility, Energoatom, said earlier in the day emergency diesel generators were providing electricity to cool the reactors, and it was trying to send a new supply of fuel to the plant.

The utility said that as of 10 a.m. Wednesday, local time, the Russian side had not let its convoy of vehicles through.

Energoatom condemned Russia, saying shelling of energy infrastructure related to the operation of nuclear power plants is the same manifestation of terrorism as direct attacks on such plants.