China has begun blanket radiation testing on seafood imports from Japan, sources familiar with the bilateral relationship said Tuesday, a move seen as pressuring Tokyo's plan to release treated radioactive water from the crippled Fukushima nuclear plant into the sea.

The measure introduced earlier this month has prolonged customs procedures, prompting some businesses in China to forego importing seafood from Japan. The Japanese foreign and farm ministries have begun discussing how to address the issue, the sources said.

Photo taken in February 2023 shows a demonstration of filleting tuna from Japan's Nagasaki during an event in Shanghai. (Kyodo)

Tokyo aims to start discharging the Fukushima water into the ocean around summer, but China vehemently opposes the plan.

On July 7, China's customs authorities said in a statement that they will "maintain a high degree of vigilance" and "take all necessary measures in a timely manner according to the development of the situation."

The blanket testing began shortly after the statement was issued, requiring about two weeks for chilled items to clear customs and about one month for frozen seafood. Radiation examinations of seafood had only been conducted in Japan, but not in China, the sources said.

Since the 2011 Fukushima nuclear accident triggered by a major earthquake and tsunami, China has banned food imports from Fukushima and nine other Japanese prefectures. If the water discharge begins, Beijing could further tighten controls on food imports from Japan.

A Japanese restaurant in Shanghai said it had no choice but to use tuna imported from Spain as seafood imports from Japan stopped last Thursday.

A Shanghai dealer told its clients it had halted the import and delivery of fresh fish from Japan on Monday. It plans to send staff abroad to find alternative procurement sources.

At the Fukushima Daiichi plant, massive amounts of radioactive water have been generated in the process of cooling melted fuel debris.

The water is treated at a processing facility to remove most contaminants, except tritium, and stored in tanks installed on the premises. But the tanks are nearing capacity, and the continuing water buildup could hamper the reactor decommissioning progress unless it is dumped into the sea.