Tokyo had announced on Tuesday that the operation into the Pacific would begin on Thursday, prompting an angry response from China and partial import bans on Japanese seafood by Hong Kong and Macau.

The operator of the plant, TEPCO, said Tuesday that it diluted a cubic metre of the waste water with around 1,200 cubic metres of seawater and allowed it to flow into position in a pipe.

This water will be tested and then from Thursday released into the sea together with more water stored at the site that will be transferred and diluted, TEPCO said in a statement.

The Fukushima-Daiichi nuclear power station was knocked out by a massive earthquake and tsunami that killed around 18,000 people in March 2011, sending three of its reactors into meltdown.

TEPCO has since collected 1.34 million cubic metres of water -- almost 540 Olympic swimming pools' worth -- used to cool what remains of the still highly radioactive reactors, mixed with groundwater and rain.

A special system has filtered out all radioactive nuclides except for tritium, levels of which will be well within safe limits, according to TEPCO.

The release has been endorsed by the UN atomic watchdog, which said it will have staff on site on Thursday for the start of the release, which is due to take several decades to complete.

China had already accused Japan of treating the ocean like a "sewer", banning imports of food from 10 of Japan's 47 prefectures even before the release and imposing strict radiation checks.

China on Tuesday summoned Japan's ambassador "to make solemn representations", while Hong Kong and Macau, both Chinese territories, announced bans on imports of "aquatic products" from the same 10 regions.

Analysts said that while China may have genuine safety concerns, its strong reaction is also at least in part motivated by its economic rivalry and frosty relations with Japan.