Ukraine’s Zaporizhzhia nuclear power plant, which is under Russian control, was rocked by shelling on Sunday, drawing condemnation from the head of the UN nuclear watchdog who said such attacks risked a major nuclear disaster.

Repeated shelling of the plant in southern Ukraine has raised concern about the potential for a grave accident just 500 km (300 miles) from the site of the world’s worst nuclear accident, the 1986 Chornobyl disaster.

More than a dozen blasts shook the Zaporizhzhia plant, which Russia took control of shortly after its February invasion of Ukraine, on Saturday evening and Sunday, the International Atomic Energy Agency (IAEA) said.

An IAEA team on the ground said there had been damage to some buildings, systems and equipment at the plant, Europe’s largest nuclear power station. The IAEA team could see some of the explosions from their windows.

“The news from our team yesterday and this morning is extremely disturbing,” UN nuclear watchdog chief Rafael Grossi said in a statement.

“Explosions occurred at the site of this major nuclear power plant, which is completely unacceptable. Whoever is behind this, it must stop immediately. As I have said many times before, you’re playing with fire!”

The Zaporizhzhia nuclear power plant has six Soviet-designed VVER-1000 V-320 water-cooled and water-moderated reactors containing Uranium 235, which has a half-life of more than 700 million years.

The reactors are shut down but there is a risk that nuclear fuel could overheat if the power that drives the cooling systems was cut. Shelling has repeatedly cut power lines.

Russia’s defense ministry said that Ukraine fired shells at power lines supplying the plant, while TASS reported some of the site’s storage facilities had been hit, quoting an official from Russian nuclear power operator Rosenergoatom.

“They shelled not only yesterday, but also today, they are shelling right now,” Renat Karchaa, an adviser to Rosenergoatom’s CEO said, adding that any artillery attack at the site posed a threat to nuclear safety.

Karchaa said the shells had been fired near a dry nuclear waste storage facility and a building that houses fresh spent nuclear fuel, but that no radioactive emissions had currently been detected, according to TASS.

Ukrainian officials did not immediately respond to a request for comment.

Both Kyiv and Moscow have accused each other of attacking the plant and risking a nuclear accident.

The facility provided about a fifth of Ukraine’s electricity before Russia’s Feb. 24 invasion, and has been forced to operate on back-up generators a number of times.