Japan's nuclear regulator has granted approval for utility Tokyo Electric Power Company, which ran the destroyed Fukushima nuclear power plant, to start releasing more than a million tonnes of radioactive water.

The global watchdog, the International Atomic Energy Agency (IAEA), said a two-year review showed Japan's plan for the release would have negligible environmental impact.

The Japanese regulator's certificate was the final step the utility required to begin the process.

However, China will increase its scrutiny of food from Japan and maintain curbs on some Japanese imports, the government said, citing Tokyo's decision to discharge treated radioactive water from the crippled Fukushima plant into the sea.

China's customs administration said in a statement it was stepping up monitoring of products including seafood and keeping curbs on produce from one-fifth of Japan's prefectures for safety reasons.

In the immediate aftermath of the 2011 disaster, China banned the importation of food and agricultural products from five prefectures. It later widened its ban to 12 prefectures, before removing two of them.

Under the Japanese plan, the water will first be filtered to remove most radioactive elements except for tritium, an isotope that is difficult to separate from water, and diluted to well below international standards.