Invading Russian forces controlled the Chernobyl zone -- the site of the world's worst nuclear disaster in 1986 -- for more than a month before retreating in late March.

"Today, Russia's illegal and unjustified aggression in Ukraine again jeopardises nuclear safety on our continent," the European Commission said in a statement.

It said Moscow's forces had "targeted and occupied Ukrainian nuclear sites, recklessly damaging the facilities."

"The unlawful occupation and the interruption of normal operations, such as preventing the rotation of personnel, undermine the safe and secure operation of nuclear power plants in Ukraine and significantly raise the risk of an accident," it said.

Russian troops have also seized the Zaporizhzhia atomic plant, Europe's largest, after an attack at the site that drew global concern.

"On the anniversary of the 1986 Chernobyl accident, we reiterate our utmost concern over the nuclear safety and security risks caused by Russia's recent actions at the Chernobyl site," the EU statement said.

"We call on Moscow to return control of the occupied Zaporizhzhia nuclear power plant to the Ukrainian authorities and refrain from any further actions targeting nuclear installations."

The head of the UN's atomic watchdog called Russia's occupation of Chernobyl "very, very dangerous" during a visit to the site on Tuesday with a team of experts to carry out radiation checks and deliver essential kit.

Ukraine says that Russian soldiers dug trenches in highly radioactive locations during their occupation of the exclusion zone.