The water discharge began at 10:18 am (0118 GMT), a TEPCO spokesman told AFP.

It follows the first discharge on August 24, when Japan began releasing some of the 1.34 million tonnes of wastewater built up since a tsunami struck the facility in 2011.

As with the first release, about 7,800 tonnes of water are expected to be discharged in the second phase.

China banned all Japanese seafood imports after the first release, which ended on September 11, despite Tokyo's insistence that it posed no health risks.

Russia, which also has frosty relations with Japan, is reportedly considering following suit on the seafood ban.

TEPCO has said the wastewater has been filtered of all radioactive elements except tritium, which is within safe levels, a view backed by the United Nation's nuclear watchdog.

China has accused Japan of using the ocean like a "sewer", an assertion echoed at the UN last week by Prime Minister Manasseh Sogavare of the Solomon Islands, who has developed close ties with Beijing.

The full wastewater release, which is expected to take decades to complete, is aimed at making space to eventually begin removing highly dangerous radioactive fuel and rubble from the plant's wrecked reactors.