Kyiv and Moscow have for months accused each other of shelling near the site, triggering fears of a nuclear catastrophe similar to the 1986 Chernobyl disaster in Soviet Ukraine.

"On our way to Kyiv for important meetings," International Atomic Energy Agency (IAEA) head Rafael Grossi wrote on Twitter, saying the need for a protection zone around the site was "more urgent than ever".

He posted his tweet with pictures showing him boarding a train with the Ukrainian Railways logo.

Russian forces have occupied the Zaporizhzhia power plant, Europe's largest, since March.

Grossi, who also visited the plant last month, has advocated setting up a security perimeter that both sides would commit not to attack.

But a decree by Putin on Wednesday stated that the Russian government would "ensure that the nuclear facilities at the plant... are integrated as federal property".

Ukrainian state nuclear agency Energoatom slammed the decree, calling it "void, absurd and inadequate".

"Zaporizhzhia NPP will continue to operate in Ukraine, in accordance with Ukrainian legislation, in the Ukrainian energy grid," Energoatom said on social media.

In a separate statement, the IAEA, which has two experts on site at the plant, said preparations were underway to restart one of the six reactors "at reduced power to produce steam and heat for the needs of the plant".

It added it would take "some time" to complete all preparations. The plant's last operating reactor was shut down on September 11.

The Zaporizhzhia facility is located near the front line in the southern Ukrainian region of the same name, one of four Russian-occupied regions Moscow formally annexed last week.

The annexations followed referendums denounced as a sham by the West and Kyiv.

In response to Grossi's proposal for a security zone, Mikhail Ulyanov, Russia's permanent representative to international organisations in Vienna, said: "We fully share this goal. The question is how it is going to be implemented."

He added: "Rafael Grossi has some practical ideas. They will be discussed tomorrow in Kyiv and next week in Russia."

Ulyanov, speaking on the sidelines of a meeting of the OPEC+ oil-producing nations in the Austrian capital, expressed doubt over whether Grossi would visit Zaporizhzhia again during his trip.

"The security situation is rather volatile," he said, adding "constant shelling" would mean another visit by Grossi would take time to prepare.

burs-jza/raz