Contaminated water with dangerously high levels of radiation is leaking from a storage tank at Japan's crippled Fukushima nuclear plant, the most serious setback to the clean up of the worst nuclear accident since Chernobyl.

The storage tank breach of about 300 tons of water is separate from contaminated water leaks reported in recent weeks, plant operator Tokyo Electric Power Co said yesterday.

The latest leak, which is continuing, is so contaminated that a person standing 50 centimeters away would, within an hour, receive a radiation dose five times the average annual global limit for nuclear workers.

After 10 hours, a worker in that proximity to the leak would develop radiation sickness with symptoms including nausea and a drop in white blood cells.

"That is a huge amount of radiation. The situation is getting worse," said Michiaki Furukawa, professor emeritus at Nagoya University and a nuclear chemist.

Embattled utility Tokyo Electric has struggled to keep the Fukushima site under control since an earthquake and tsunami caused three reactor meltdowns in March 2011.

Japan's Nuclear Regulation Authority classified the latest leak as a level 1 incident, the second lowest on an international scale for radiological releases, a spokesman said yesterday.

But it is the first time Japan has issued an INES rating for Fukushima since the meltdowns. Following the quake and tsunami, Fukushima was assigned the highest rating of 7, when it was hit by explosions after a loss of power and cooling.

A Tokyo Electric official said that workers monitoring storage tanks appeared to have failed to detect the leak.

"We failed to discover the leak at an early stage and need to review not only the tanks but also our monitoring system," he said.

Continued contaminated water leaks from Fukushima have alarmed Japan's neighbors South Korea and China.

Tokyo Electric, also known as Tepco, has been criticized for its failure to prepare for the disaster and accused of covering up the extent of problems.

Tepco said it did not believe that water from the latest leak had reached the ocean, about 500 metres away.

A puddle that formed near the leaking tank is emitting a radiation dose of 100 millisieverts an hour about 50 centimeters above the water surface, a Tepco official said.

The government said this month it will step up its involvement in the plant's cleanup, following Tepco's admission, after months of denial, that leaked contaminated water had reached the ocean.

Tepco has also struggled with worker safety. This month, 12 workers decommissioning the plant were found to have been contaminated by radiation. The utility has not yet identified what caused those incidents, which only came to its notice when alarms sounded as workers prepared to leave the site.

A South Korean Foreign Ministry spokesman confirmed yesterday that Seoul had asked Japanese officials to publicly explain what they were doing to stop contaminated water reaching the Pacific Ocean.

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