The plan, approved two years ago by the Japanese government as crucial to decommissioning the plant operated by Tokyo Electric Power Company (Tepco) (9501.T), has also faced criticism from local fishing groups, who fear reputational damage and a threat to their livelihood.

"I promise that we will take on the entire responsibility of ensuring the fishing industry can continue to make their living, even if that will take decades," Kishida said on Monday.

Foreign ministry spokesman Wang Wenbin said in July that Japan had shown selfishness and arrogance, and had not fully consulted the international community about the water release.

South Korean activists have also protested the plan, although Seoul has concluded from its own study that the water release meets international standards and said it respects the IAEA's assessment.

The water was used to cool the fuel rods of Fukushima Daiichi after it melted down in an accident caused by a huge tsunami in 2011 that battered Japan's eastern coast.

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