The head of the International Atomic Energy Agency (IAEA), Rafael Grossi, said on Monday his agency was preparing an expert mission to the Zaporizhzhia nuclear plant, the largest in Europe and now occupied by Russian forces.

Grossi said on Twitter the visit was arranged after Ukraine had "requested" it.

But Ukraine's nuclear agency, Energoatom on Tuesday accused Grossi of lying and said it did not greenlight the visit.

"Ukraine did not invite Grossi to visit the Zaporizhzhia plant and refused to let him make such a visit in the past. The visit to the plant will only become possible when Ukraine takes back control of the site," Energoatom wrote on Telegram.

"IAEA Director General Rafael Grossi is lying again," Energoatom said.

"We consider this declaration a new attempt to gain access to the Zaporizhzhia power plant to legitimise the presence of the occupiers and approve their actions."

It added that communication between the plant and the IAEA was lost after the Russians took over the site in March, and that server data would be given to the IAEA when the Ukrainian operator is in charge again.

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.

Russian forces took control of the plant in southern Ukraine at the beginning of March, soon after Moscow's February 24 invasion.