The Zaporizhzhia nuclear power plant in southern Ukraine was targeted in drone attacks, the U.N. nuclear watchdog said Sunday.

Officials from the International Atomic Energy Agency (IAEA) present at the power station — which has been under Russian occupation since March 2022 shortly after the Kremlin's full-scale invasion of Ukraine began — confirmed that the drones targeted one of the site’s six reactor buildings where there was surveillance and communication equipment.

The officials added that Russian troops, who were posted on the roof of one of the buildings, “engaged what appeared to be an approaching drone” and that “an explosion near the reactor building” followed.

There were three direct drone hits, said Rafael Mariano Grossi, the head of the IAEA, adding that “although the damage at unit 6 has not compromised nuclear safety, this was a serious incident that had the potential to undermine the integrity of the reactor’s containment system.”

It’s not clear who launched the drones. While Russian officials alleged the attack had been conducted by Ukrainian forces, Kyiv officials denied in local media that they had anything to do with it.

It’s the first time since November 2022 that the power plant was directly targeted by military action, Grossi said, adding that it is a clear violation of the five principles which he established in May 2023 with Russian and Ukrainian authorities to guarantee the safety and security of the facility.

IAEA officials on site reported that “outside a laboratory, they saw blood stains next to a damaged military logistics vehicle, indicating at least one casualty,” the agency wrote.

“This is a major escalation of the nuclear safety and security dangers,” Grossi said. “Such reckless attacks significantly increase the risk of a major nuclear accident and must cease immediately.”

Zaporizhzhia is the largest nuclear power station in Europe with six reactors. It currently does not produce electricity after all its reactors were shut down.

There has been renewed shelling around the plant in recent weeks, which led to the station losing connection to one of its last remaining back-up power lines last week.