This picture taken by Tokyo Electric Power Company on Monday shows contaminated water that leaked from a water tank at its Fukushima nuclear power plant. An estimated 300 metric tons of radioactive water is believed to have leaked from the plant. Agence France-Presse

An estimated 300 metric tons of highly radioactive water is believed to have leaked from one of the hundreds of storage tanks at Japan's tsunami-crippled nuclear power plant, the operator said on Tuesday as it battled the latest toxic water threat.

Tokyo Electric Power Company said the leak was believed to be continuing on Tuesday at Fukushima. It has not figured out how or where the water leaked, but suspects it did so through a valve connected to a gutter around the tank.

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 were pumping out the puddle and the remaining water in the tank and will transfer it to other containers.

The water's radiation level, measured about 50 centimeters above the puddle, is about 100 millisieverts per hour - five times the annual exposure limit for plant workers, he said.

"This means you are exposed to the level of radiation in an hour that a nuclear plant worker is allowed to be exposed to in five years," he told reporters.

The company later said it had identified which tank was leaking but had yet to find the spot from where it was leaking.

"We have instructed TEPCO to find the source of contaminated water ... and to seal the leakage point," an official from the Japanese Nuclear Regulation Authority said.

"We have also instructed them to retrieve contaminated soil to avoid a further expansion of toxic water, and to strengthen monitoring of the surrounding environment."

There were no significant changes in radiation levels outside the plant, he added.

TEPCO admitted it was possible the toxic water could contaminate groundwater and flow into the Pacific Ocean "in the longer term" but said it was working to avoid such a situation.

So far, four tons of the spilled water had been retrieved since Monday evening when TEPCO started the recovery operation, the company said.

TEPCO has faced a growing catalogue of incidents at the plant including several leaks of radioactive water, more than two years after the worst nuclear disaster in a generation triggered by a huge quake and tsunami in March 2011.

The company - which faces huge cleanup and compensation costs - has struggled with a massive amount of radioactive water accumulating as a result of continuing water injections to cool reactors.

The embattled utility in July admitted for the first time that radioactive groundwater had been leaking outside the plant. This month it started pumping it out to reduce leakage into the Pacific.

The problems have led the Japanese government and its nuclear regulator to say they would get more directly involved in the cleanup at Fukushima.

While no one is officially recorded as having died as a result of the meltdowns of Fukushima's reactors, large areas around the plant had to be evacuated.

A spokesman for the South Korean foreign ministry confirmed on Tuesday media reports that the country's government has asked Japanese officials to explain the leakage of contaminated water into the Pacific Ocean.

AFP-AP-Reuters