A long-awaited visit by UN experts to the Russian-occupied Zaporizhzhia nuclear power plant would be successful if it led to the "demilitarisation" of the facility, the head of Ukraine's state nuclear company said on Thursday.

Energoatom chief Petro Kotin, speaking to Reuters at a Ukrainian-controlled checkpoint on the road to the plant, said Ukrainian authorities were making "all efforts" to switch back on the plant's fifth reactor after it shut down due to shelling early on Thursday.

Kotin said the fact that inspectors from the International Atomic Energy Agency (IAEA) and its director Rafael Grossi had been able to visit the plant on Thursday after months of negotiations marked progress. But he added that Russia had tried to propagandise the visit by excluding independent journalists.

Kotin condemned as "illegal" the presence of a representative from Russia's state nuclear company Rosatom at the tour given to the IAEA on Thursday.

"He was the main person (on the tour), and actually the (plant's) director was just beside him," Kotin said.

The plant in southern Ukraine has been controlled by Russian forces since soon after the Russian attack on Ukraine in February but it is still operated by Ukrainian staff.

Russia and Ukraine accused each other of trying to sabotage the IAEA mission to the plant. Conditions at the nuclear plant, Europe's largest, have been unravelling for weeks, with Moscow and Kyiv regularly trading blame for shelling in the vicinity and fuelling fears of a Chornobyl-style radiation disaster.

Kotin said Ukraine's top priority was to demilitarise the plant and the surrounding area to avoid a nuclear accident.

"If this mission helps to do that, then it will be successful," he said, adding his hope that the IAEA visit would mark a "step forward" toward that goal.

IAEA chief Rafael Grossi was due to depart on Thursday, but several other members of the delegation would stay on at the plant through Saturday, Kotin said.

Shortly after the interview with Reuters, Energoatom said in a Telegram post that Grossi, who led the body's 14-person delegation to the plant, had left the facility. It said five mission members had remained at the plant, and would likely stay through Saturday. (Reuters)