Japan's nuclear watchdog said it is concerned that more storage tanks at the wrecked Fukushima nuclear plant will spring leaks.

The announcement follows the discovery that highly contaminated water is leaking from one of the hastily built containers.

Japan is set to raise its warning about the severity of a toxic water leak at Fukushima, its nuclear watchdog said.

The deepening crisis at the Fukushima plant will be upgraded from a level 1 "anomaly" to a level three "serious incident" on an international scale for radiological releases.

That will mark the first time Japan has issued a warning on the International Nuclear Event Scale (INES) since three reactor meltdowns after the massive quake in March 2011.

Water still leaking from the plant is so contaminated that a person standing close to it for an hour would receive five times the annual recommended limit for nuclear workers.

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

China said it was "shocked" to hear that Fukushima was still leaking contaminated water two years after the disaster and urged Japan to provide information "in a timely, thorough and relevant way".

"We hope 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," China's Foreign Ministry said in a statement.

A maximum level 7 was declared at the battered plant after explosions led to a loss of power and cooling two years ago, confirming Fukushima as the worst nuclear accident since Chernobyl a quarter of a century earlier.

Contaminated water with dangerously high levels of radiation is leaking from a storage tank at Fukushima, the plant's operator, Tokyo Electric Power Co said.

The leak was classified as an "anomaly" earlier this week.

The Nuclear Regulation Authority’s impending assessment upgrade came in a document posted on the agency's website today, with formal adoption to follow a meeting that is being held by the authority's commissioners, the NRA spokesman said.

"Judging from the amount and the density of the radiation in the contaminated water that leaked ... a level 3 assessment is appropriate," the document said.