The director general of the International Atomic Energy Agency (IAEA) called Thursday for immediate action to ensure the safety of nuclear power plants in Ukraine in order to prevent a possible nuclear accident.

"We must do everything to prevent a nuclear accident at any of these nuclear facilities, which would only add to the terrible suffering we are already witnessing in Ukraine," Rafael Grossi said in a video posted on his official Twitter account.

Touching on the importance of the safety of the nuclear facilities, Grossi said their security is "of central and vital concern to the IAEA."

He noted that the IAEA has been making progress in establishing a nuclear safety and security protection zone around the Zaporizhzhya site for which Ukraine has been receiving the agency’s assistance since the beginning of September.

"While the world has been focused on the dangerous situation at Zaporizhzhya, Europe’s largest nuclear power plant, we must not forget the country’s other nuclear facilities," Grossi said.

Grossi highlighted that “yesterday, for the first time ever, all of Ukraine’s four operational nuclear power plants -- Zaporizhzhya, Rivne, South Ukraine and Khmelnytskyi -- lost external power and were disconnected from the grid."

These nuclear power plants had to use emergency diesel generators for electricity, he noted.

Describing the incident as "deeply worrying," he said it would have been "unimaginable just months ago."

Ukraine's state nuclear energy company Energoatom announced Wednesday that all power units in four nuclear power plants in the country had automatically disconnected due to a decrease in frequency amid Russian strikes in multiple regions.

Grossi said Ukrainian authorities also requested the agency’s assistance and support for five nuclear power facilities in the country, namely Zaporizhzhya, Rivne, Khmelnytskyi, South Ukraine and Chornobyl.

He added that the IAEA assistance has begun.