Ukraine’s Zaporizhzhia nuclear power plant, Europe’s largest atomic energy station, spent several hours operating on emergency diesel generators Monday after losing power for the seventh time since Russia’s invasion began last year, the UN nuclear watchdog said.

“This morning’s loss of all off-site power demonstrates the highly vulnerable nuclear safety and security situation at the Zaporizhzhia Nuclear Power Plant, Rafael Grossi, head of the International Atomic Energy Agency, said in a statement.

Hours later, national energy company Ukrenergo said on Telegram that it had restored the power line that feeds the plant.

For Grossi, it was a reminder of what is at stake at the Russian-occupied plant, which, as the first ever nuclear power plant located on the front lines of an all-out war, has seen shelling precariously close by.

“As I’ve said repeatedly, this simply can’t go on,” he said. “We’re playing with fire. We must act now to avoid the very real danger of a nuclear accident in Europe, with its associated consequences for the public and the environment.”

The plant’s six Soviet-built VVER reactors, which are protected by reinforced concrete capable of reducing damage if hit by an errant shell or rocket, have been shut down.

But a disruption in the electrical supply could disable cooling systems that are essential for the reactors’ cooling functions even in their idled state. Emergency diesel generators, which officials say can keep the plant operational for 10 days, can be unreliable and their fuel supplies easily interrupted by military clashes.

Energoatom, Ukraine’s state nuclear company, blamed Russian shelling for the loss of the last high-voltage transmission line to the plant in Russian-occupied southern Ukraine, about 500 kilometers from Kyiv — though those claims have not been independently verified.

The facility is “on the verge of a nuclear and radiation accident,” Energoatom warned. Once the power line was restored, Energoatom described the situation as “stabilized.

The new blackout came amid reports from the IAEA that staff shortages at the plant could become dire. Since mid-May, according to the agency, residents of Enerhodar, the company city of the Zaporizhzhia plant, have been voluntarily evacuating, depleting an unknown but possibly critical number of personnel.

Russian officials have begun training for a planned evacuation from the plant of 3,100 staff and their families, a representative of Energoatom told the Associated Press last week. The official said that most of those were plant personnel who had signed contracts with various affiliates of Russia’s nuclear agency Rosatom.

Before the war, the plant employed around 11,000 people, some 6,000 of whom remain at the site and in the surrounding town of Enerhodar, the representative said.

About 500 Russian troops are stationed at the site, while at least 1,500 others are based in the nearby city of Enerhodar, the representative told AP.