Russian forces occupying the Zaporizhzhia nuclear plant in southern Ukraine are restricting access of staff to the plant, Ukraine's nuclear agency says.

It warns that Russia wants to disconnect power units supplying the Ukrainian electricity grid and is planning a "provocation".

The plant in southern Ukraine is Europe's largest nuclear plant and both sides have accused each other of dangerous shelling.

Turkish President Recep Tayyip Erdogan on Thursday affirmed Ankara's support for Kyiv and warned of the risk of a nuclear power plant disaster, during his first visit to Ukraine since the Russian invasion.

"As we continue our efforts for a solution, we have been and continue to be on the side of our Ukrainian friends," he said at a joint press conference with his Ukrainian counterpart Volodymyr Zelenskyy and UN Secretary-General Antonio Guterres in Lviv.

"We are concerned. We don't want to have another Chornobyl," he said, referring to the Russian-occupied Zaporizhzhia nuclear plant in southern Ukraine.

"We have to tell it like it is: any potential damage to Zaporizhzhia would be suicide," Guterres said, as Kyiv and Moscow accused each other of bombing the site.

Zelenskyy said the United Nations must ensure the security of the Zaporizhzhia nuclear power plant occupied by Russian forces, following the meeting in western Ukraine, in a drive to spur diplomatic efforts to help end the war.

"Particular attention was paid to the topic of Russia's nuclear blackmail at the Zaporizhzhia NPP (nuclear power plant). This deliberate terror on the part of the aggressor can have global catastrophic consequences for the whole world," Zelenskyy wrote on the Telegram messaging app.

"Therefore, the UN must ensure the security of this strategic object, its demilitarisation and complete liberation from Russian troops."