The UN's nuclear watchdog said Thursday it was planning to visit the Zaporizhzhia nuclear plant, in a Russian-occupied part of Ukraine, despite opposition from Kyiv.

"This mission is not a matter of wanting or wishing, it is an obligation on the side of Ukraine and on the side of the IAEA," said the Director General of the UN body, Rafael Grossi, ahead of a board meeting of the International Atomic Energy Agency (IAEA) in Vienna this week.

The current situation was untenable, he added, as essential maintenance work was continually being postponed, vital equipment not delivered and the risk of an accident increasing.

The Zaporizhzhia nuclear plant in southeastern Ukraine is the largest in Europe and is now occupied by Russian forces. 

Russia threatened on May 19 to cut Ukraine off from the Zaporizhzhia plant unless Kyiv paid Moscow for the electricity produced. 

In 2021 -- well before Russia's invasion of Ukraine -- the Zaporizhzhia plant represented 20 percent of Ukraine's annual electricity production and nearly half of all nuclear power produced in Ukraine.

In March, Russian soldiers took control of the plant in the city of Energodar, separated by the Dnipro river from the regional capital, Zaporizhzhia, which is still under Kyiv's control.

Grossi said in his statement that he was very concerned at "the extremely stressful and challenging working conditions under which Ukrainian management and staff are operating the plant".

"This is why IAEA safety and security experts must go" to the site," he insisted, adding that he was "actively working" to organise an IAEA-led international mission to the Zaporizhzhia plant to carry out essential safety work,

However officials in Kyiv on Thursday reiterated opposition to such a visit.

By visiting the site, the IAEA would only "legitimise the occupation of the power plant caused by the presence of nuclear terrorists", Ukraine's .nuclear agency Energoatom said on Telegram.

"In what way the IAEA mission can get to the power plant is still unknown. Nobody from Ukraine invited them there," Energoatom added in its statement.

"Are they going to come... through Russia and then Crimea or other temporarily occupied Ukrainian territories under the protection of the Russian military without the will of Ukraine?!" it added.

Such a visit would only be possible once Ukraine had regained control of the site, the nuclear operator insisted, citing the example of Chernobyl which the IAEA visited at the end of April, only after the withdrawal of Russian soldiers.