China will stop all seafood imports from Japan, escalating tensions between the two nations as Japan begins a contentious release of treated wastewater from the wrecked Fukushima nuclear power plant. 

The move to “comprehensively” pause purchases of aquatic products from Japan goes into effect Thursday, the Chinese customs office said in a statement, effectively making good on warnings this week to take “necessary steps” to safeguard food safety and the marine environment.

China is the largest single export market for Japanese exports of fish, crustaceans and aquatic invertebrates, with Japanese companies exporting almost 72 billion yen ($496 million) worth to China last year. Beijing has been the most vocal critic of Japan’s plan, which Prime Minister Fumio Kishida has insisted will meet safety standards.

Japan has also stressed that similar releases of wastewater are relatively commonplace. A two-year review by the International Atomic Energy Agency found Japan’s strategy would have a negligible impact on people and the environment.

Even so, China blasted the release as “selfish and irresponsible.” The customs authority said the suspension of imports was intended to prevent radioactive contamination risks. 

Chinese Foreign Ministry spokesman Wang Wenbin reiterated Thursday that Japan failed to prove the discharge is safe and harmless to people and the environment. By dumping the water, Japan is spreading the risk to the rest of the world, Wang said at a regular press briefing in Beijing. 

Earlier in the day, China’s Ministry of Ecology and Environment said it would step up radiation monitoring in the nation’s sea areas and watch for any impact from the release.

China already bans food imports from 10 prefectures around the Fukushima plant. The Hong Kong government this week announced a ban on seafood imports from those same prefectures, four of which are landlocked.

Chinese seafood stocks rallied after the announcement from the Chinese customs authority. Apart from Japan, China also buys seafood from other countries including Ecuador, Russia and Canada.

Alex Capri, a senior lecturer at the NUS Business School in Singapore and a research fellow at the Asia-based Hinrich Foundation, sees Beijing’s latest measure as retaliation for Japan’s deeper security ties to the US and Tokyo’s move to join Washington in controlling exports of chipmaking equipment. 

“Clearly, the writing’s on the wall and going forward, Japan’s posture is going to be more de-risking and more strategic decoupling from China,” he said. “I think we’re going to see Japanese techno-nationalism, old-fashioned economic nationalism and security interests continuing to realign with Washington and the West.”

Chinese state media reported that local officials in Shanghai conducted a surprise raid of supermarkets and Japanese grocery stores Thursday to see if they sold food from Fukushima and other regions that have been banned. No violations were found. 

The process to release the treated wastewater — equivalent in volume to about 500 Olympic-size swimming pools — from the Fukushima site will take place over a period of at least 30 years.