All three nuclear power plants still under Ukrainian control disconnected automatically from the electricity grid on Wednesday after fresh Russian air strikes hit the country, Ukraine's nuclear operator said.

The strikes activated emergency protocols at the Rivnenska, Pivdennoukrainska and Khmelnytska plants and "as a result... all reactors were automatically disconnected" from the electricity grid, Energoatom said.

The plants are currently not generating electricity "into the domestic energy system," the operator said.

The plants would re-supply electricity as soon "as the operation of the power system is normalised," it added.

The radiation level at the nuclear plants and surrounding territories is unchanged, Energoatom said.

In the south, the Russian-occupied Zaporizhzhia nuclear plant "went into full blackout mode. All diesel generators are in operation," Energoatom said.

Diesel generators are back-ups, supplying the plant's essential systems in case it gets cut off from the grid.

Still, radiation levels remain normal at Zaporizhzhia, Europe's largest atomic facility.

Russia and Ukraine have traded blame for attacks near several nuclear plants in Ukraine since Russian troops invaded on February 24.

Kyiv has said Russian authorities have detained dozens of staff at the Zaporizhzhia plant, endangering operations and raising concerns of a nuclear disaster.