Japan won approval from the U.N.'s nuclear watchdog on Tuesday for its plan to release treated radioactive water from the tsunami-wrecked Fukushima plant into the ocean, despite fierce resistance from Beijing and some local residents.

After a two-year review, the International Atomic Energy Agency (IAEA) said Japan's plans were consistent with global safety standards and that they would have a "negligible radiological impact to people and the environment".

"This is a very special night," IAEA chief Rafael Grossi told Prime Minister Fumio Kishida before handing him a thick blue folder containing the final report.

Grossi later told reporters at the Japan National Press Club, where he was met by a small group of protesters, that he would seek to allay lingering concerns and would station IAEA staff at the Fukushima plant to monitor the release.

"We have to recognise that such a thing has not happened before," he said, adding that Japan would have the final say on the release, which is due to span 30 to 40 years.

Japan's government maintains the process is safe as it has treated the water - enough to fill 500 Olympic-sized swimming pools - used to cool the fuel rods of the Fukushima plant after it was damaged by the earthquake and resulting tsunami.

Japan has not specified a date to start the water release pending official approval from the national nuclear regulatory body for Tokyo Electric Power (Tepco) 9501.T, whose final word on the plan unveiled in 2021 could come as early as this week.

Japanese fishing unions have long opposed the plan, saying it would undo work to repair reputations after several countries banned some Japanese food products after the 2011 disaster.

A petition from the regions around the plant has garnered more than 250,000 signatures since the proposal was first made.

Some neighbouring countries have also complained over the years about the threat to the marine environment and public health, with Beijing emerging as the biggest critic.

"Japan will continue to provide explanations to the Japanese people and to the international community in a sincere manner based on scientific evidence and with high level of transparency," Kishida said as he met with Grossi.

Through its embassy in Japan, Beijing said the IAEA report cannot be a "pass" for the water release and called for the plan's suspension.

TREATED WATER

Japan says the water has been filtered to remove most radioactive elements except for tritium, an isotope of hydrogen that is difficult to separate from water. The treated water will be diluted to well below internationally approved levels of tritium before being released into the Pacific.

In a presentation given to foreign journalists in China last month, Japanese officials said tritium levels in the treated water are lower than those found in waste water regularly released by nuclear plants around the world, including in China.

The officials said they had made multiple and repeated attempts to explain the science behind Tokyo's stance to Beijing, but their offers had been ignored.

China on Tuesday said Japan's comparison of the tritium levels in the treated water and waste water was "completely confusing concepts and misleading public opinion".

Grossi will visit the Fukushima plant on Wednesday. After his Japan trip, he will head to South Korea, where consumers have been snapping up sea salt and other items ahead of the water release.

He is also expected to visit New Zealand and the Cook Islands in a bid to ease concerns over the plan, according to media reports.

READ MORE: Explainer: How Japan plans to release Fukushima water into the ocean