China will closely track and assess the impacts of Japan's "extremely selfish and irresponsible" plan to discharge nuclear-contaminated water into the ocean, the National Nuclear Safety Administration said.

Japan's plan "prioritizes its own interests over the long-term well-being of humanity", the administration said in a media release on Thursday, the same day that Japan is set to start releasing radioactive water into the Pacific Ocean as part of its plan to decommission the crippled Fukushima Daiichi Nuclear Power Plant.

Japanese Prime Minister Fumio Kishida gave the final go-ahead for the plan to gradually release more than 1 million metric tons of "treated and diluted" radioactive water from the plant into the ocean at a Cabinet meeting on Tuesday.

"We urge the Japanese government to listen to the voice of the international community and effectively dispose of the nuclear-contaminated water in a scientific, safe and transparent manner under strict international supervision," the administration said.

It said the administration has carried out radiation monitoring in China's sea areas in the past two years, and will beef up the work in a move to "effectively safeguard the country's national interests and people's health".

Based on its monitoring work in 2021 and 2022, the administration has figured out the current background information of the ocean radiation environment in its sea areas, it said.

With no abnormal concentration of artificial radionuclides in seawater and marine organisms, it said, the overall levels of such radionuclides in China's sea areas are within the range of fluctuations in previous years.

It said the administration is organizing ocean radiation monitoring for this year. It will continue to strengthen the monitoring work, closely tracking and assessing the potential impact of the discharge of the nuclear-contaminated water from Japan.