The leaking of radioactive water into the Pacific Ocean has prompted Japan's nuclear watchdog to propose that the risk level of at the Fukushima nuclear power plant is raised from "an anomaly" to a "serious incident."
The call was made after it emerged that approximately 300 tonnes of highly radioactive water has leaked from one of hundreds of steel tanks around the wrecked Fukushima Dai-ichi plant.
The leak is the fifth in the past year involving the tanks, and Japan's nuclear watchdog is concerned further potentially disastrous leaks could come.
China has said it is "shocked" that Japan's Fukushima nuclear plant is still leaking radioactive water two years after it was destroyed by an earthquake and tsunami, the Chinese Foreign Ministry has said.
China "hopes that the Japanese side can earnestly take effective steps to put an end to the negative impact of the after-effects of the Fukushima nuclear accident," the Foreign Ministry said in a statement.
The Japanese nuclear watchdog proposed raising the rating of a leakage of highly radioactive water at the crippled Fukushima nuclear power plant from "an anomaly" to a "serious incident."
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Tepco, the company behind the Fukushima nuclear power plant, suspects a rat might have caused Monday's outage.
The Japanese firm released a picture of a rodent carcass which it says probably triggered a short-circuit in a switchboard.
In March 2011 a massive tsunami caused a meltdown at the plant on the country's north east coast.
A power outage has left four fuel storage pools at Japan's tsunami-damaged Fukushima nuclear plant without fresh cooling water for more than 15 hours. The plant's operator it was trying to repair a broken switchboard that might have caused the problem.
Tokyo Electric Power Company said that pool temperatures were well within safe levels at the plant, and that pools would remain safe for at least four days without fresh cooling water.
People living in the areas contaminated by radioactive material released by the Fukushima Daiichi nuclear accident two years ago have a higher risk of developing certain cancers over their lifetime, the World Health Organisation said.
The United Nations agency said: "This health risk assessment concludes that no discernible increase in health risks from the Fukushima event is expected outside Japan.
"With respect to Japan, this assessment estimates that the lifetime risk for some cancers may be somewhat elevated above base-line rates in certain age and sex groups that were in the areas most affected."
The UN health organisation is due to publish its findings on the health impact of the Fukushima nuclear disaster in Japan today.
The in-depth report will give the World Health Organisation's assessment of risks from radiation exposure to people all over the world, as well as in the immediate vicinity.
Three reactors at Japan's Fukushima Daiichi nuclear plant experienced meltdowns after a 15-metre tsunami cause by a major earthquake disabled the plant's power supply.
No-one has died as a result of the accident, but more than 100,000 people were evacuated to avoid exposure to radiation.
Reuters reports that Tokyo Electric says there is no problem with the cooling system at the Fukushima nuclear plant after an earthquake in Japan and that there has been no irregularities in radiation levels.
Japan has approved the restarting of two nuclear reactors reactors despite mass public opposition. They will be the first to come back on line after they were all shut down following the Fukushima crisis.
Prime Minister Yoshihiko Noda has backed the restarts for some time, despite public concerns over safety after the big earthquake and tsunami crippled the Fukushima plant.
Anti-nuclear campaigners fear it could open the door to more restarts among Japan's 50 nuclear power reactors, which formerly provided around a third of Japan's electricity demand.