Russia says Ukraine has struck the Zaporizhzhia nuclear power station controlled by Russian forces three times and demanded the West respond, although Kyiv says it had nothing to do with the attacks.
The International Atomic Energy Agency (IAEA), which has experts at the site, said it was the first time the nuclear plant, Europe's largest, was directly targeted since November 2022.
It said the attack had endangered nuclear safety.
Russian forces took control of the plant in 2022 shortly after their full-scale invasion of Ukraine.
Both Moscow and Kyiv have repeatedly accused each other of risking a nuclear accident by attacking the plant.
Russia's state nuclear corporation, Rosatom, said Ukraine attacked the plant three times on Sunday with drones, first injuring three near a canteen, then attacking a cargo area and then the dome above reactor six.
"Zaporizhzhia nuclear power station has been subjected to an unprecedented series of drone attacks, a direct threat to the safety of the plant," Rosatom said.
"The radiation levels at the plant and the surrounding area have not changed."
A Ukrainian intelligence official said Kyiv had nothing to do with any strikes on the station and suggested they were the work of Russians themselves.
"Russian strikes, including imitation ones, on the territory of the Ukrainian nuclear power plant ... have long been a well known criminal practice of the invaders," spokesperson for Ukraine's HUR Main Intelligence Directorate Andriy Usov said.
Reuters was unable to immediately verify battlefield accounts from either side.
The nuclear plant has six Soviet-designed VVER-1000 V-320 water-cooled and water-moderated reactors containing Uranium 235 and also has spent nuclear fuel at the facility.
Four reactors are in cold shutdown while one is shut down for repair and another is in so-called "hot shutdown", according to the plant.
Russian Foreign Ministry spokeswoman Maria Zakharova urged world leaders to condemn the act of "nuclear terrorism".
Zakharova asked how many more times Ukraine would endanger nuclear safety at the plant before Western leaders took action.
The IAEA said its experts had confirmed three drone attacks and that Russian troops engaged what appeared to be a drone approaching reactor six.
"This is a major escalation of the nuclear safety and security dangers facing the Zaporizhzhia Nuclear Power Plant," IAEA Director General Rafael Grossi said in a statement.
"Such reckless attacks significantly increase the risk of a major nuclear accident and must cease immediately."
Grossi said attacking a nuclear power plant was "an absolute no-go".
"Although the damage at unit six has not compromised nuclear safety, this was a serious incident that had the potential to undermine the integrity of the reactor’s containment system," he said.
It comes as Russia launched more than two dozen attack drones on Ukraine, hitting critical infrastructure in the central Zhytomyr region and damaging logistics facilities in the south, officials said.
The Zvyahel city council in Ukraine's Zhytomyr region urged people to stay indoors after Russia's air attack on a critical infrastructure object resulted in air pollution.
"Russia struck an infrastructure object of the community overnight," the city council said on the Telegram messaging app on Monday.
"There are no casualties among the civilian population.
"Currently, there is a direct threat of air pollution, so it is recommended to stay indoors with closed windows."
Officials provided no further details.
Russia has intensified its drone and missile strikes against Ukrainian energy facilities in recent weeks, causing significant damage and threatening Ukrainians with a repeat of the blackouts they experienced in the first year after Russia invaded in February 2022.
