Rafael Grossi said that “patchy” communication from the Zaporizhzhia facility and his organisation’s inability to visit the site were deeply concerning.
“What is at stake is extremely serious and extremely grave and dangerous,” he said.
The IAEA is unsure whether the plant, which is now in Russian-controlled territory but is being run by Ukrainian staff, is receiving all the parts it needs to operate properly, as its supply of equipment has been affected by the war.
“Every principle of nuclear safety has been violated,” Mr Grossi said of the site, adding that his organisation urgently needs to check that Zaporizhzhia’s nuclear material is being safeguarded.
He called on both Ukraine and Russia to allow experts to reach the plant to assess the situation as soon as possible.
The power plant was experiencing a “catalogue of things that should never be happening in any nuclear facility”, the IAEA’s director-general stressed.
“And this is why I have been insisting from day one that we have to be able to go there to perform this safety and security evaluation, to do the repairs and to assist as we already did in Chernobyl,” he said.
Mr Grossi said the agency’s presence “will be a deterrent to any act of violence against this nuclear power plant.
“So I’m pleading as an international civil servant, as the head of an international organisation, I’m pleading to both sides to let this mission proceed.”
Fears about a nuclear disaster similar to the 1986 accident at Chernobyl grew when Russia captured the plant shortly after it invaded Ukraine on 24 February.
Reports suggested that the Russians damaged an administrative building while seizing the facility. The reactors were not affected by the attack.
Mr Grossi visited Chernobyl on April 27 and tweeted that the level of safety was “like a ‘red light’ blinking”.
But he said that the IAEA set up “an assistance mission” at Chernobyl at that time “that has very, very successful so far”.
He is now pleading for a similar visit to Zaporizhzhia “to prevent a nuclear accident from happening”, Mr Grossi said.