Japan has lost its biggest seafood market -- China -- as it begins its release of treated radioactive water from the destroyed Fukushima nuclear power plant.

Japan exported about $600 million (€552.8 million) worth of aquatic products to China in 2022, making it the biggest market for Japanese exports, with Hong Kong second. Sales to China and Hong Kong accounted for 42 percent of all Japanese aquatic exports in 2022, according to government data.

Japan started releasing the water into the Pacific Ocean on Thursday, prompting China to announce an immediate blanket ban on all aquatic products from Japan, reports Reuters.

Chinese officials said they are highly concerned about the risk of radioactive contamination from Japan's food and agricultural products.

The Japanese government signed off two years ago on a plan to release the water, and it was given a green light by the UN nuclear watchdog last month. The discharge is a key step in decommissioning the Fukushima Daiichi plant, which was destroyed by a tsunami in 2011.

Plant operator Tokyo Electric Power said it had not identified any abnormalities in the water since the release started.

However, China reiterated its opposition to the plan and said the Japanese government had not proved that the water discharged would be safe.

Tokyo has in turn criticized China for spreading "scientifically unfounded claims."

Separately from China, Hong Kong and Macau announced their own ban starting Thursday, which covers Japanese seafood imports from 10 regions.

The release of water will take about 30 years.