Chinese foreign ministry spokesman Zhao Lijian urged Japan to "act in a responsible manner". The company has been using a makeshift system of pumps and piping to inject water into damaged reactor vessels to keep melted uranium fuel rods cool.
Japan's Prime Minister Yoshihide Suga told a ministerial meeting that disposing of the water was an "inevitable task" in the decades-long process of decommissioning the nuclear plant.
"The United States is aware that the GOJ examined several options related to the management of the treated water now being stored onsite at the Fukushima Daiichi site", the US Department of State said in a press release Tuesday, referring to the government of Japan.
Japan's government argues that the release will be safe because the water has been processed to remove nearly all radioactive elements and will be diluted.
Tomoaki Kobayakawa, president of plant operator TEPCO, said it would "take thorough measures to prevent bad rumours" affecting local industries.More news: State trooper among 6 shot in Texas shootings
The decision also prompted regional opposition even before it was official, with South Korea's foreign minister on Monday expressing "serious regret".
"To safeguard worldwide public interests and Chinese people's health and safety, China has expressed grave concern to the Japanese side through the diplomatic channel", Zhao said Monday.
Japanese Prime Minister Yoshihide Suga said on Tuesday that his government has made a decision to discharge contaminated radioactive wastewater in Fukushima Prefecture into the sea amid domestic and global opposition.
The US State Department, however, said Japan had been "transparent about its decision, and appears to have adopted an approach in accordance with globally accepted nuclear safety standards". The space of the containing tanks is expected to reach capacity next year.
Either method would be "in line with well-established practices all around the world", he added.More news: Britain's Prince Philip Has Died at Age 99
China said it has conveyed its "serious concern" to Japan, calling on Prime Minister Yoshihide Suga's government to make a cautious decision to protect the public interest of global society as well as the health and safety of Chinese citizens. The dilution process won't remove tritium from the water but could reduce other radionuclides to safe levels, it said.
The activist groups are also calling on the South Korean government to take strong countermeasures, including filing a complaint with the International Tribunal of the Law of the Sea, and increase pressure on Japan in solidarity with international civic organizations.
Some scientists say the long-term impact on marine life from low-dose exposure to such large volumes of water is unknown.
But lawyers and local plaintiffs who have filed class-action lawsuits for damages against TEPCO and the government demanded the "immediate withdrawal" of the decision.More news: Ukraine's Zelensky on frontline as Merkel urges Putin to pull back troops