Tokyo Electric Power (TEPCO) said it started releasing 7,800 tonnes of water that had been used to cool reactors that went into meltdown after a 2011 deadly tsunami.

"This one (release) is estimated to finish in about 17 days," a TEPCO spokeswoman told AFP.

From late August, the utility gradually began releasing the 540 Olympic swimming pools' worth of wastewater that is being stored on the campus of the Fukushima Daiichi power plant.

The facility was running out of space to build more water tanks, and TEPCO needs to clear the area for the much more hazardous task of removing radioactive fuel and rubble from three stricken reactors.

Japan argues that the water being released is harmless and heavily diluted with seawater. It is also being released gradually over decades.

The International Atomic Energy Agency and many leading economies have sided with Japan.

But China, later joined by Russia, criticised the release and banned all Japanese seafood imports, saying that Japan was polluting the environment.

Experts from the IAEA and other agencies, including those from China, have surveyed the environmental impact of the release, including by taking water and fish samples.

The Chinese ban has particularly harmed scallop fishermen in the northern Hokkaido region, some 500 kilometres (300 miles) north of the Fukushima plant, who rely on Chinese factories for shelling the molluscs.

TEPCO and other Japanese businesses were swamped with crank calls from China after the initial release, but now the number is negligible, the TEPCO spokeswoman said.