IAEA chief Rafael Grossi led a team of inspectors to the Russian-controlled plant that has been frequently shelled in recent weeks raising fears of a nuclear incident.

"It is obvious that the plant and physical integrity of the plant has been violated several times," Grossi told reporters after he returned with part of his team to the Ukrainian-controlled area.

"I worried, I worry and I will continue to be worried about the plant," he said, adding, however, that the situation was "more predictable" now.

"We have spent there four or five hours. I have seen a lot, and I have my people there, we were able to tour the whole site," Grossi said about the long-anticipated inspection.

He said that part of his 14-strong mission to the plant will stay at the facility "until Sunday or Monday, continuing with the assessment".

"We are going to continue presence there, some of my experts. We have a lot of work there, to do analysis of some technical aspects," Grossi said.

He did not specify how many people will be staying at the facility, adding that "we could draw up a number of questions and initial observations, initial assessments, and they are going to dig deeper into that so that we could have a report".

"This morning the situation was pretty difficult," he said.

"There were moments where fire was obvious, heavy machine gun, artillery, mortars two or three times, we were very concerned," Grossi said, describing the security situation during his visit.

But he noted that the mission received "splendid support from the UN security team".

"I think we showed that international community is there, could be there and we will be continuing this," Grossi said.

The Zaporizhzhia nuclear plant -- Europe's largest nuclear facility -- has faced repeated shelling in recent weeks, with Kyiv and Moscow blaming each other for the attacks, raising concerns of a possible incident.

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