A new radioactive water leak has been discovered at Japan's crippled Fukushima nuclear plant and may have flowed into the Pacific Ocean, its operator said Thursday, according to Japanese news agencies.

Tokyo Electric Power (TEPCO) said the highly radioactive water had leaked at the Fukushima No. 1 plant from a different storage tank to the one where a similar leak was found in August, Jiji and Kyodo news agencies reported.

It was not clear how much water had leaked from the 450-ton tank. But at an early morning press conference Thursday the plant operator admitted that "contaminated water may well have flowed into the sea", according to Kyodo.

The report quoted a TEPCO official as saying: "We are sorry for causing anxiety."

It is the second time in as many months that highly radioactive water has leaked from storage tanks surrounding the stricken plant, which was heavily damaged by a March 2011 earthquake and tsunami.

In August, some 300 tons of toxic water was discovered to have leaked from a separate tank, with part of it believed to have flowed into the Pacific Ocean.

TEPCO said it had determined that the contaminated water from the latest leak -- which contained highly radioactive doses of strontium -- had accumulated within barriers surrounding one of the tanks.

The barriers were installed to block water from spreading when a leak occurs in the storage tanks at the plant.

TEPCO said it had informed the Japanese government of the latest leak and the country's Nuclear Regulatory Authority has ordered the company to stem the flow and remove any contaminated soil, Kyodo reported.

TEPCO has long struggled to control waste water at the plant ever since the earthquake and tsunami caused some of Fukushima's reactors to melt down.

The company poured thousands of tonnes of water onto runaway reactors to keep them cool, and continues to douse them.

But TEPCO has so far disclosed no clear plan for disposing of the huge amounts of stored polluted water.

According to the Kyodo report, the tanks that the water is currently being stored in are made from steel sheets joined by bolts but TEPCO plans to replace those containers with more watertight welded tanks.