Russia is using Europe's largest nuclear power plant as a base to store weapons including "missile systems" and shell the surrounding areas of Ukraine, an official with Kyiv's nuclear agency has said.

Located on the Dnipro river in southeastern Ukraine, the Zaporizhzhia nuclear plant has been under Russian control since the early weeks of Moscow's invasion, though it is still being operated by Ukrainian staff.

The president of Ukrainian nuclear agency Energoatom said Friday in a televised interview that the situation was "extremely tense", with up to 500 Russian soldiers controlling the site. 

"The occupiers bring their machinery there, including missile systems, from which they already shell the other side of the river Dnipro and the territory of Nikopol," Petro Kotin said, referring to the city across the water.

"They physically control the perimeter. The occupiers' heavy machinery and trucks with weapons and explosives remain on the territory of the Zaporizhzhia nuclear power plant," he said.

"The pressure on the occupiers to leave the territory of the plant is insufficient," he added, before taking aim at the International Atomic Energy Agency. 

"(The IAEA) is playing some political games, balancing between Russia and Ukraine," he said.

The IAEA has said it needs to visit the plant to conduct essential maintenance work.

Kyiv has opposed such a visit, with Energoatom declaring that an IAEA trip to the site would only legitimise its occupation by "nuclear terrorists".

On Thursday, IAEA chief Rafael Grossi once again stressed the importance of "being able to travel to the (Zaporizhzhia nuclear power plant) to conduct essential safety, security and safeguards activities", according to a statement.

He also "reiterated his growing concern about the severe and challenging conditions facing staff at the ZNPP and the impact of such conditions on the safety and security of the plant".

The agency has not been able to visit the plant since the Russian invasion of Ukraine on February 24.

The Zaporizhzhia region where the plant is located is largely under Russian control, and Moscow-backed separatists said Thursday they are planning to stage a referendum on joining Russia this year.