Japanese officials are worried that China, the biggest buyer of its seafood exports, may halt purchases of those items after Tokyo begins releasing treated radioactive water from its crippled Fukushima nuclear plant into the sea.

The UN nuclear watchdog this week gave Japan the greenlight to begin discharging more than a million metric tons of water – enough to fill 500 Olympic swimming pools – used to cool the plant's fuel rods after it was wrecked by a 2011 tsunami.

The planned release of the water from the plant north of Tokyo has faced opposition at home and abroad despite Japanese assurances that it is safe after being filtered to remove most isotopes.

China was the biggest buyer of Japanese seafood exports last year, even though it is among several countries that restrict imports from some Japanese regions because of the nuclear accident. The nation is also the fiercest critic of Japan's planned release of the water saying it threatens marine life and human health.

Beijing has not said what action it will take if the release goes ahead but it has warned Japan that it must "bear all the consequences" of its action. The release is due to start in coming weeks and take up to 40 years.

The Foreign Ministry said on Thursday Japan did not fully consult the international community on the release and China would pay close attention to developments and would assess any possible effects to protect consumers.

Three Japanese government officials and a ruling party lawmaker, speaking on condition of anonymity to Reuters news agency, said they expected China to expand restrictions on Japanese seafood. Two of the officials said that could include a blanket ban.

"We think they may enforce a total ban on Japanese maritime products," one of the officials said. "They want to punish Japan economically for this."

"For China, Japanese maritime product imports make up a small share of their market ... but for Japan this is a big market."

China accounted for 22.5 percent of Japanese seafood exports, worth 87 billion yen (US$604 million), followed by Hong Kong with 19.5 percent. (Reuters)