KYIV, Ukraine — The head of the United Nations nuclear watchdog visited Europe’s largest atomic power plant Thursday in southern Ukraine, where a recent dam burst and the start of a counteroffensive in the war by Kyiv’s forces have heightened safety risks.

Ukraine’s national nuclear energy company, Energoatom, announced the visit in a Telegram post.

Rafael Mariano Grossi, the International Atomic Energy Agency chief, met Tuesday in Kyiv with Ukraine President Volodymyr Zelenskyy to discuss concerns about the Russia-occupied Zaporizhzhia Nuclear Power Plant.

The IAEA has repeatedly expressed alarm about the facility, which is one of the 10 biggest in the world, amid fears of a potential nuclear catastrophe. The agency has officials stationed at the plant, which is still run by its Ukrainian staff.

The plant has repeatedly been caught in the crossfire since Russia launched its war on Ukraine in February 2022 and seized the facility shortly after.

Last week, the destruction of the Kakhovka dam in southern Ukraine’s partially Russian-occupied Kherson region added a new concern. The dam, further down the Dnieper River, helped keep water in a reservoir that cools the plant’s reactors.

The plant’s reactors have been shut down for months, but it still needs power and qualified staff to operate crucial cooling systems and other safety features.

NATO Secretary-General Jens Stoltenberg said Thursday that areas of the 600-mile front line in Ukraine are witnessing “fierce fighting” after Kyiv’s long-anticipated launch of its counteroffensive using Western-supplied weapons.

He said Ukrainian forces “are making gains.” Even so, Western analysts and military officials have cautioned that driving the Kremlin’s forces out of Ukraine could take a long time.

Ukraine has so far been “testing” Russian defenses, looking for weaknesses, according to Mykhailo Podolyak, an adviser to Zelenskyy.

“We haven’t yet started our counteroffensive as such,” he claimed in televised remarks.

He noted that Ukrainian troops were launching simultaneous attacks in many directions, seeking to sow panic among the Russian troops.

The Russian Defense Ministry, meanwhile, said its military used long-range air-launched cruise missiles to strike Ukrainian drone-making facilities. It claimed all the targeted facilities were hit but didn’t mention where they were or any other details.

It was not possible to verify battlefield claims by either side.

Russian forces are keeping up their aerial attacks on Ukraine. They launched four cruise missiles and 20 Iranian Sahed exploding drones overnight, the Ukrainian air force said.

Ukrainian officials said the nation’s air defenses downed all the drones and one cruise missile.