"So far we have been able to confirm that the first releases of these waters do not contain any radionucleide at levels that would be harmful," Rafael Grossi told AFP during a visit to Stockholm.

Twelve years after one of the world's worst nuclear accidents, Japan began releasing the wastewater into the Pacific Ocean last week, as it gradually discharges around 540 Olympic swimming pools' worth of water over several decades.

"The beginning has been according to what we were expecting ... but we will continue (to monitor) ... until the last drop is released," Grossi said.

The IAEA said on August 24 that its independent analysis of the tritium concentration in the diluted water being discharged was "far below the operational limit of 1,500 becquerels per litre."

That limit was in turn much lower than the Japanese national safety standard.

Japan has repeatedly insisted the wastewater will be harmless, but the move has elicited fears among local fishermen and sparked anger in China, which has suspended its seafood imports from Japan.

Grossi also commented on his agency's cooperation with Iran, saying that the reinstallation of cameras at nuclear sites was progressing too slowly.

Tehran in March vowed to reactivate surveillance devices which were disconnected in June 2022 amid deteriorating relations with the West.

"We've been trying to have our cameras reinstalled. We started that work but it is not going at the pace I would like and expect," Grossi said.

"It has been very, very slow and we would like this to improve."

The IAEA is due to issue a new report on developments in Iran's nuclear programme soon.

"We are looking into some clarifications Iran should provide us about findings of uranium traces," he said.

"It is an ongoing process that has a lot of room for improvement."