The UN nuclear watchdog is "developing the modalities" for an international mission of experts it hopes to send to the Russian-held nuclear power plant at Zaporizhzhia in Ukraine, Europe's largest, its chief Rafael Grossi said on Monday.

"We are developing the modalities to dispatch such a mission; other considerations should not prevent this essential international mission from taking place," Grossi said in a statement to the International Atomic Energy Agency's 35-nation Board of Governors, adding that Ukraine had requested it.