Concerns have surrounded the safety of the power plant -- Europe's largest nuclear power station -- since it was seized by Russian forces in March 2022.

Its six reactors have been shut down, unprecedented for a plant of its size.

"On a technical point of view, we have many questions and we are trying to address these one by one with the administration," Grossi said in a press conference in Kyiv.

Grossi heads the International Atomic Energy Agency (IAEA) that has had a monitoring team on the ground since September 2022.

But its experts have not been able to inspect every part of the power station, said Grossi, who will visit the plant on Wednesday.

At times "we weren't granted the access that we were requesting for certain areas of the facility," Grossi said.

"We were allowed partial access," he said, but "there are still some parts of the plant we have not been able to visit."

The Russian operator has started granting increasing access, Grossi said in Kyiv on Tuesday after a meeting with Ukrainian President Volodymyr Zelensky.

The nuclear chief also raised the alarm over a drastic reduction in staff numbers at the plant, which he plans to discuss on Wednesday with the Russian operator.

Only around 4,500 staff are employed on site -- down from 11,500 before the war -- triggering safety worries.

Grossi said he had fresh concerns over staffing levels after the Russian operator barred pro-Ukrainian staff from working at the plant.

The IAEA has repeatedly warned of persistent nuclear safety and security risks at the site.

Grossi nevertheless hailed a "gradual increase in the way that both (Russian and Ukrainian) sides are following what the IAEA says."