Ukraine’s Zaporizhzhia nuclear plant, now under Russian control, was rocked over the weekend by a renewed barrage of shelling over the weekend, prompting the UN nuclear watchdog to warn that such attacks risk a major disaster.

More than a dozen blasts shook the plant, which is Europe’s biggest nuclear station, on Saturday evening and Sunday, the International Atomic Energy Agency said. The barrage of shells — some falling near reactors and damaging a radioactive waste storage building — shattered a period of relative calm.

Russia and Ukraine on Monday traded blame for the explosions at the plant, which has been under Russian control since soon after it invaded the country on February 24 but is across the Dnieper River from areas controlled by Kyiv.

IAEA head Rafael Grossi said news of the blasts was extremely disturbing.

“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!” he said in a statement.

Ukraine’s President Volodymyr Zelensky urged NATO members to guarantee protection from “Russian sabotage” at nuclear facilities. The head of Russia’s state-run nuclear energy agency, Rosatom, said it had discussed the shelling with the IAEA and said there was a risk of a nuclear accident, the official Russian newswire TASS reported.

Rosatom has controlled the facility through a subsidiary since President Vladimir Putin in October ordered Russia to formally seize the plant and transfer Ukrainian staff to a Russian entity. Kyiv says the transfer of assets amounts to theft.

As of Monday, various news agencies could not verify which side was responsible for the blasts. The attacks hit a cooling pond, a cable to one reactor and a bridge to another, according to an IAEA team on the ground citing information provided by plant management.

“We were fortunate a potentially serious nuclear incident did not happen. Next time, we may not be so lucky,” Grossi said in a statement late on Sunday, describing the situation as a “close call.”

“We are talking meters, not kilometers,” he said.

The IAEA team plans to conduct damage assessments at the plant early this week, Grossi said in the statement. But Rosenergoatom, the Rosatom structure that oversees nuclear power plant operation, said there would be curbs to what the IAEA was allowed to see.

“They interpret their mandate as having no limits. This is not so,” Renat Karchaa, an adviser to Rosenergoatom’s CEO, told TASS.

“If they want to inspect a facility that has nothing to do with nuclear safety, access will be denied,” he said.

Repeated shelling of the plant during the war has raised concern about a grave disaster in the country that suffered the world’s worst nuclear accident, the 1986 Chernobyl meltdown.

Radiation levels remained normal and there were no reports of casualties, the IAEA said. While there was no direct impact on nuclear safety and security systems, “the shelling came dangerously close”, Grossi said.

The Zaporizhzhia nuclear plant provided a fifth of Ukraine’s electricity before Russia’s invasion, and has been forced to operate on back-up generators a number of times. It has six Soviet-designed VVER-1000 water-cooled and water-moderated reactors running on uranium 235.

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