Japan began releasing wastewater from the crippled Fukushima nuclear plant into the Pacific Ocean on Thursday, prompting a furious China to ban all seafood imports from its neighbor.

The start of the discharge of around 540 Olympic swimming pools' worth of water over several decades is a big step in decommissioning the still highly dangerous site 12 years after one of the world's worst nuclear accidents.

Japan has repeatedly insisted the wastewater will be harmless, with its position backed by the UN atomic watchdog the International Atomic Energy Agency.