The head of the United Nations nuclear watchdog agency has condemned drone strikes at the Zaporizhzhia Nuclear Power Plant, saying “such reckless attacks significantly increase the risk of a major nuclear accident and must be stopped immediately.”

At least three drones detonated at the plant on Sunday, according to inspectors from the U.N. International Atomic Energy Agency who are stationed at the facility. One strike left scorch marks on the roof of the containment building housing one of the plant’s six nuclear reactors, the agency said. Another hit outside a laboratory building. The location of the third drone strike was not included in the agency’s statement.

The facility, Europe’s largest nuclear power plant, is precariously perched on the eastern banks of the Dnipro river near the frontline dividing the warring armies, and has been a source of concern almost since the start of the war. It is the first time that a nuclear facility has been occupied by an invading army and repeated crises at the plant have prompted global alarm over the rising risks of a radiological disaster.

“The experts reported hearing explosions and rifle fire on the site throughout the day,” the agency said in a statement Sunday night. “Additionally, the I.A.E.A. team heard several rounds of outgoing artillery fire from near the plant.”

The U.N. agency did not speculate on who was responsible for the attacks. Ukraine and Russia each blamed the other for the strikes.

Rafael Mariano Grossi, the director general of the U.N. International Atomic Energy Agency, said in a statement that there were “no indications of damage to critical nuclear safety or security systems,” but international inspectors at the facility observed “minor superficial scorching to the top of the reactor dome roof” of one unit.