"We are now finally moving after six months of... efforts. The IAEA is moving into the Zaporizhzhia nuclear power plant," IAEA chief Rafael Grossi told reporters in Kyiv before setting off.

"I am really very conscious of the relevance of this moment but we are ready. The IAEA is ready. We will be reporting back after the mission. We are going to be spending a few days there," he added.

The nuclear plant, which is Europe's biggest, was captured by Russian forces in March shortly after the start of Moscow's invasion of Ukraine.

Ukraine has accused Russia of deploying soldiers and storing ammunition and armoured vehicles there.

Russia and Ukraine have accused each other of being behind recent shelling around the plant that has raised international fears of a possible incident.

Ukraine was the site of the world's worst nuclear catastrophe in 1986, when a reactor at the northern Chernobyl plant exploded and spewed radiation into the atmosphere.

Ukraine had initially feared an IAEA visit at Zaporizhzhia would legitimise the Russian occupation of the site, before finally supporting the idea of a mission.

But Ukraine has insisted that the mission should travel there from Ukrainian-held territory.

Meeting Grossi on Tuesday, Ukrainian President Volodymyr Zelensky said he was "very thankful" for the visit and warned that the situation around the plant was "extremely menacing".