"We have expressed serious concerns and opposition to its plan, which could affect our people's health and security as well as the ocean environment", the official was quoted as saying by the Yonhap news agency.
The official said that Japan made the decision to flush the radioactive water into the sea without consulting its neighbours. Russia, China and South Korea are worried that the disposal of a million tonnes of radioactive water could damage the ecosystem.
Prime Minister Fumio Kishida said during a visit to the power plant on Sunday that the sight of all the cisterns containing treated wastewater made him realize that its release could no longer be delayed.
Japan said in April that the wastewater would be released gradually starting in 2023. The global nuclear watchdog, IAEA, said nuclear power plants routinely dump decontaminated wastewater, although the amounts stored at the Fukushima plant make it a "unique and complex" case.