Over the past 24 hours, the agency was informed of "a major increase in drone activity with more than 160 UAVs recorded flying in the vicinity of the sites," the International Atomic Energy Agency (IAEA) said in a statement.

"While the IAEA's teams reported no direct impact on nuclear safety, Director General (Rafael) Grossi expressed deep concern about such military activities," it said.

Activities "have intensified over the past several days, posing significant risks to nuclear safety and security," it added.

It named the Khmelnitsky, Rivne and South Ukraine operational nuclear plants and the Chernobyl site, largely deserted since the plant there suffered a catastrophic meltdown in 1986.

It said the IAEA team at Chernobyl was informed that a fire near the site -- caused last week by a drone crash according to Ukrainian authorities -- was brought under control and extinguished by early this week.

In the statement, Grossi urged "all parties to exercise maximum restraint".

Russia pummelled Kyiv for hours on Thursday, firing hundreds of drones and dozens of missiles in a deadly attack that further shredded hopes of a halt to Moscow's grinding invasion.

Russia and Ukraine have repeatedly traded accusations of targeting nuclear plants throughout the war, triggered when Moscow launched its full-scale offensive in February 2022.

Russian troops seized the Zaporizhzhia nuclear power plant in southern Ukraine, which is in a cold shutdown for safety. They continue to control it.

They also captured the defunct Chernobyl facility in the early months of the invasion, before retreating weeks later.