JAPAN has started releasing radioactive water from the demolished Fukushima nuclear power plant into the Pacific Ocean, prompting China to announce a ban on Japanese seafood, reports Reuters.

China is "highly concerned about the risk of radioactive contamination brought by Japan's food and agricultural products," said China Customs.

The Japanese government signed off on the plan two years ago and it was given a green light by the UN nuclear watchdog last month. The discharge is a key step in decommissioning the Fukushima Daiichi plant after it was destroyed by a tsunami in 2011.

However, China reiterated its firm opposition to the plan and said the Japanese government had not proved that the water discharged would be safe.

"The Japanese side should not cause secondary harm to the local people and even the people of the world out of its own selfish interests," said its foreign ministry statement.

Tokyo has in turn criticised China for spreading "scientifically unfounded claims."

It maintains the water release is safe, noting that the International Atomic Energy Agency (IAEA) has also concluded that the impact it would have on people and the environment was "negligible".