Japanese authorities are set to dramatically increase the level of their warning about the severity of a toxic water leak at the Fukushima nuclear power plant, the country's nuclear watchdog says.

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, a spokesman for Japan's Nuclear Regulation Authority (NRA) said on Wednesday.

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 earthquake and tsunami of March 2011.

On Tuesday, officials confirmed that about 300,000 litres of contaminated water had leaked from a tank designed to hold overflows from the site.

TEPCO said that because the currently leaking tank is about 100m from the coastline, the leak does not pose an immediate threat to the sea. But Hideka Morimoto, a watchdog spokesman, said water could reach the sea via a drain gutter.

Four other tanks of the same design have had similar leaks since last year. The incidents have shaken confidence in the reliability of hundreds of tanks that are crucial for storing what has been a continuous flow of contaminated water.

TEPCO spokesman Masayuki Ono said the leaked water seeped into the ground after largely escaping piles of sandbags added to a concrete barrier around the tank.

Workers pumped out the puddle and the remaining water in the tank to other containts, in a desperate effort to prevent it from escaping into the sea. Not all of the contaminated water was contained, however, Ono said.

Dangerously high radiation levels

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 in a year.

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 Company (TEPCO) said on Tuesday. The leak was classified as an "anomaly" earlier this week.

The NRA's impending assessment upgrade came in a document posted on the agency's website on Wednesday, 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.

The leak, which has not been plugged, is so contaminated that a person standing 0.5m 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.

Each one-step INES increase represents a tenfold increase in severity.

In its statement on Wednesday, the Japanese nuclear agency also said that it is concerned about other storage tanks, of which there are hundreds, at the wrecked plant springing leaks.