Russia and Ukraine again trade blame for shelling at the Zaporizhzhia nuclear plant.

At least a dozen shells exploded at a large nuclear plant in southern Ukraine on Sunday, Ukrainian and Russian authorities said, damaging equipment in attacks that the United Nations’ nuclear watchdog agency called “extremely disturbing.”

Russian and Ukrainian nuclear energy authorities each blamed the other side’s forces for the strikes, the latest to hit the Zaporizhzhia Nuclear Power Plant, which is the largest in Europe. The attacks have raised fears of a serious nuclear accident at the plant, which is occupied by Russian forces, although so far there have been no reports of any leak of radiation.

“Explosions occurred at the site of this major nuclear power plant, which is completely unacceptable,” the director general of the U.N. International Atomic Energy Agency, Rafael Mariano Grossi, said in a statement. “Whoever is behind this, it must stop immediately.”

Energoatom, Ukraine’s state nuclear company, said on Sunday that the shelling continued “all morning” and damaged equipment including water storage tanks and a steam purge generator system. “At least 12 hits were recorded,” the company said in a post on the Telegram messaging app.

Russia’s nuclear agency, Rosenergoatom, blamed Ukraine’s military for the shelling. An adviser to the company’s director general, Renat Karchaa, told the state-owned Russian news agency Tass that 15 shells had landed.

“Any artillery strikes at a nuclear power plant jeopardize nuclear safety,” he said, adding that a building where nuclear fuel is stored had been hit.

The extent of the damage to the facilities was not immediately clear.

Russian forces seized the Zaporizhzhia plant in March, stationing troops and military equipment there. Over the summer, Russian troops shelled neighboring cities from the grounds of the plant, according to the Ukrainian authorities. At the same time, shelling has repeatedly struck the plant’s facilities, including storage tanks for spent nuclear fuel, and the complex has been occasionally disconnected from external power because of attacks that have damaged electricity lines.

Both sides have blamed each other for the strikes. As a safety measure, all of the reactors have been cycled down.

The shelling on Sunday came after weeks of what the U.N. nuclear agency described as relative calm at the plant. Mr. Grossi renewed his appeals for the plant to be made into a demilitarized zone. In recent weeks, he has held talks with both the Russian and Ukrainian leaders to promote the plan, but without apparent success.