Plant operator TEPCO reported the leak in early February at a part of the site that processes radioactive water.

It said no sign of contamination had been detected outside the plant, which suffered a devastating meltdown in 2011.

Grossi, on a three-day tour of Japan that included a visit to Fukushima, described the incident as "a splash from one side of the facility".

"It concerns a small amount of water, initially estimated at five cubic metres (5,000 litres), then revised to 1.5 cubic metres of water," he told reporters.

"It was a minor incident" of the kind that can take place on a big industrial site, the IAEA chief said, adding that there were "no consequences".

The event "has nothing to do with the process of the... discharge of water from the plant", he said -- referring to the gradual release of 540 Olympic swimming pools' worth of treated and diluted wastewater which began in August.

"This was an incident in another place, a different part of the facility," Grossi added.

The water release has been endorsed by the IAEA, and TEPCO says all radioactive elements have been filtered out except for tritium, levels of which are within safe limits.

But China and Russia have criticised the release into the Pacific Ocean and banned Japanese seafood imports, saying that Japan is polluting the environment.

"Compared to last year, when discharges of water began, I saw a big change for the better" in discussions with officials and students in the Fukushima region, Grossi said.

"Last year, there was concern, uncertainty, there was doubt about whether the process would have consequences negative for the environment," he said.

"This year (there) were expressions of satisfaction" that "the treated water has been discharged without any traceable presence of, or very, very low presence of tritium."

The Fukushima plant was wrecked by a huge earthquake and tsunami in 2011 that killed 18,000 people.

It was one of the worst nuclear disasters in history and the clean-up operation is expected to take decades, with the most dangerous part -- removing radioactive fuel and rubble from three stricken reactors -- yet to begin.

kh/kaf/lb

TEPCO - TOKYO ELECTRIC POWER