China has been increasing its surveillance of Japanese nationals following Tokyo's decision to release treated radioactive water from the crippled Fukushima nuclear power plant, sources close to the matter said Sunday.

Beijing has investigated the backgrounds of individuals involved in negotiations over the water discharge and a revised counterespionage law took effect in July, broadening the scope of what constitutes spying under Chinese law.

Some political experts have said that the Communist Party may have tried to detain Japanese citizens living in China, with the leadership under President Xi Jinping having become increasingly vigilant about the activities of foreign nationals within the country.

A large screen in Beijing displays President Xi Jinping on Aug. 24, 2023. (Kyodo)

There are more than 100,000 Japanese nationals living in China, and concern has grown among the international community about Beijing's stance, which appears to treat foreign citizens as potential hostages, the experts said.

In April 2021, Japan decided it would discharge the treated radioactive water into the Pacific Ocean "in around two years." Prime Minister Fumio Kishida's government gave the green light to begin the water release on Aug. 24, triggering a strong backlash from China.

Japan has claimed that the discharge is safe, given that the treated water is diluted to reduce tritium levels to less than one-40th of the concentration permitted under national safety standards before being released into the sea.

China, however, has criticized Japan for allowing the discharge of "nuclear-contaminated water" from the plant, which was wrecked in the wake of a devastating earthquake and tsunami in March 2011.

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