“It’s obvious that… the physical integrity of the plant has been violated several times. By chance or by deliberation, we don’t have the elements to assess that, but this is the reality that we have to recognize,” Grossi told reporters after returning to Ukrainian-controlled territory on Friday.

“I worried, I worry and I will continue to worry about the plant until we have a situation that’s more stable, that’s more predictable,” he added.

Grossi, who headed the team of IAEA experts that arrived at the Zaporozhye nuclear power plant on Thursday, said that he “saw a lot” during the visit, personally inspecting some of the “key areas” at the facility, including emergency systems, diesel generators and control rooms.

The atomic agency was “not going anywhere” from the plant now, he assured reporters, reiterating that the UN nuclear watchdog plans to establish a permanent presence at the facility.

The official also said that some experts from his team will remain at the station until Sunday or Monday to “dig deeper” and collect more data for the report.

Grossi acknowledged that the security situation at the plant was “difficult” during his visit. “There were moments where fire was obvious – heavy machine gun, artillery, mortars,” he said, adding that on two or three occasions things got “very concerning” for the UN team.

Zaporozhye nuclear power plant has been under Russian control since March, but has continued to be operated by the Ukrainian staff. The IAEA chief has praised the “incredible degree of professionalism” of those employees, saying that he saw them “calm and moving.”

Moscow accused Kiev forces of repeatedly targeting the nuclear power plant, Europe’s largest, in August. It warned that those attacks could trigger a disaster that would eclipse the 1986 Chernobyl disaster, with radioactive material potentially reaching Poland, Slovakia, Romania, Moldova, Belarus, and even Germany.

Ukraine has claimed that Russian forces have turned the power plant into a military base and that they’ve been shelling the facility themselves to pin the blame on Kiev.

Russia has repeatedly denied those accusations and urged that international inspectors visit the site so that they could assess the situation for themselves.