Japan began discharging the treated contaminated water from the stricken plant earlier on Thursday in an operation it insists is safe but has generated a fierce backlash from China.

The release has also been deemed safe by the International Atomic Energy Agency, but Beijing has banned food imports from 10 Japanese prefectures, with Hong Kong following suit.

"The ocean is the common property of all humanity, and forcibly starting the discharge of Fukushima's nuclear wastewater into the ocean is an extremely selfish and irresponsible act that ignores international public interests," Beijing's foreign ministry said in a statement.

Japan "did not prove the legitimacy" of the plan or the "long-term reliability of the nuclear wastewater purification equipment", it said.

Tokyo also "did not prove the authenticity and accuracy of the nuclear wastewater data, (and) did not prove that ocean discharge is harmless to the marine environment and human health".

"What the Japanese side has done is to push the risks onto the whole world (and) pass on the pain to future generations of human beings", the statement said.

"By treating the release of the wastewater as a fait accompli, the Japanese side has simultaneously placed itself in the international dock."

In 2011, three reactors at the Fukushima-Daiichi facility in northeastern Japan went into meltdown following a massive earthquake and tsunami that killed around 18,000 people.

Since then, plant operator TEPCO has collected 1.34 million cubic metres of water contaminated as it cooled the wrecked reactors, along with groundwater and rain that has seeped in.

The beginning of the discharge of around 540 Olympic swimming pools' worth of water over several decades is a big step in decommissioning the still highly dangerous site.

Beijing said in its statement that "the handling of Japan's Fukushima wastewater is an important matter of nuclear security, (whose) impacts cross national borders".

"The Japanese side should not cause secondary harm for local people or even the people of the world out of its own self-interest."