The U.N. atomic watchdog agency urged Russia and Ukraine on Tuesday to establish a “nuclear safety and security protection zone” around the Zaporizhzhia power plant amid mounting fears the fighting could trigger a catastrophe in a country still scarred by the Chernobyl disaster.

In a report following a visit by an inspection team last week, the International Atomic Energy Agency said “shelling on site and in its vicinity should be stopped immediately to avoid any further damages to the plant and associated facilities, for the safety of the operating staff and to maintain the physical integrity to support safe and secure operation.”

“This requires agreement by all relevant parties to the establishment of a nuclear safety and security protection zone” around the plant, it said.

IAEA Director-General Rafael Grossi, who led the inspection visit, was due to brief the U.N. Security Council later Tuesday on his findings.

Shelling continued around Europe’s largest nuclear plant on Tuesday, a day after it was again knocked off Ukraine’s electrical grid and put in the precarious position of relying on its own power to run its safety systems.

Normally the plant relies on power from the outside to run the critical cooling systems that keep its reactors and its spent fuel from overheating. A loss of those cooling systems could lead to a meltdown or other release of radiation.

“For radiation protection professionals, for the Ukrainian and even the Russian people, and those of central Europe, this is a very worrying time — and that’s an understatement,” said Paul Dorfman, a nuclear safety expert at the University of Sussex in England.

Russian-installed officials accused Ukrainian forces of shelling Enerhodar, the city where the plant is situated, while the Ukrainians said Kremlin forces attacked the city of Nikopol, across the Dnieper River from the power station.

The Ukrainian mayor of Enerhodar, Dmytro Orlov, reported a powerful blast in the city around midday. The explosion left the city of 53,000 cut off from its power and water supplies. It wasn’t immediately clear what caused the blast.

World leaders have called for the demilitarization of the plant, which has been occupied by Russian forces since the early days of the war but is being run by Ukrainian engineers.

In its report, the IAEA did not assign blame for the shelling at the plant. The agency has sought to keep out of the political fray.

It did note that on several occasions, the plant lost, fully or in part, its off-site power supply because of military activity in the area. The U.N. agency said a backup power supply line should be reestablished and asked that “all military activities that may affect the power supply systems end.”

In addition, the IAEA warned that the Ukrainian staff operating the plant under Russian military occupation is “under constant high stress and pressure, especially with the limited staff available” — a situation that could “lead to increased human error with implications for nuclear safety.”

It recommended that “an appropriate work environment, including family support,” be reestablished.

The IAEA also said the staff is not being given unrestricted access to some parts of the plant and must get permission from the Russian occupying forces to reach the cooling ponds where spent fuel is kept. Grossi expressed concern that that could hamper the staff’s response in an emergency.

The report said the team saw Russian military personnel, vehicles and equipment at various locations, including several military trucks on the floor of two turbine halls. It called for “the removal of vehicles from areas that could interfere with the operation of safety and security systems and equipment.”

Two inspectors from the IAEA mission remained at the plant, a decision welcomed by Ukrainian presidential adviser Mykhailo Podolyak.

“There are Russian troops now who don’t understand what’s happening, don’t assess the risks correctly,” Podolyak said. “There is a number of our workers there, who need some kind of protection, people from the international community standing by their side and telling (Russian troops): ‘Don’t touch these people, let them work.’”

On Monday, the IAEA said Ukrainian authorities reported that the plant’s last transmission line linking it to the nation’s power grid was disconnected to allow workers to put out a fire caused by shelling.

Ukrainian Energy Minister Herman Halushchenko told Ukrainian television: “Any repairs are impossible at this point — there are ongoing hostilities around the plant.”

In the meantime, the plant’s only remaining operational reactor will “generate the power the plant needs for its safety and other functions,” the IAEA said.

Mycle Schneider, an independent analyst in Canada on nuclear energy, said that means the plant was probably functioning in “island mode,” or producing electricity for its own operations.

“Island mode is a very shaky, unstable and unreliable way to provide continuous power supply to a nuclear plant,” Schneider said. He said that “many if not most islanding attempts fail.”

The Zaporizhzhia plant has diesel emergency backup generators to produce power to run the place if the outside source is disrupted. But Schneider said the plant’s operators may have decided to go into island mode first.

If the plant turns to the diesel generators as a last resort and they fail, the reactor and the spent fuel could rapidly overheat, he said.

Experts say the reactors at Zaporizhzhia are designed to withstand natural disasters and even plane crashes, but the unpredictable fighting has repeatedly threatened to disrupt the cooling systems. Ukraine in 1986 was the site of the world’s worst nuclear accident, the explosion at Chernobyl.

Ukrainian intelligence reported that residents of Enerhodar were fleeing the city out of fear. Ukraine’s Deputy Prime Minister Iryna Vereshchuk said Russia should organize safe corridors for women and children living nearby.

“People en masse are reaching out to us for help. They are trying to leave to the dangerous territory, but there are no corridors,” Vereshchuk told Ukrainian TV.

Meanwhile, gunfire and explosions were heard Tuesday afternoon in the Russian-occupied city of Berdyansk in southastern Ukraine, with Russia’s state-run media reporting that the car of the Kremlin-installed “city commandant” had been blown up. The RIA Novosti news agency said that the official, Artem Bardin, was in serious condition and that a shootout followed the assassination attempt.

The agency quoted Russian-backed local officials as saying they had launched a manhunt for the “Ukrainian saboteurs” responsible.

In the southern Kherson region, occupied by the Russians since early on in the war, Ukrainian forces continued their counteroffensive. A pontoon bridge was blown up overnight and a command center was hit, as well as two checkpoints, Ukrainian authorities said.