Highly radioactive waste water from a crippled Fukushima nuclear plant has leaked to the Pacific, its operator said Tuesday, promising to prevent similar incidents.

Tokyo Electric Power Co. (TEPCO) said it believes 150 litres (40 US gallons) of waste water including highly harmful strontium, linked with bone cancers, has spread to the open ocean.

The announcement came a day after TEPCO said it found 45 tonnes of waste water pooled around the leaky water-treatment system at the Fukushima Daiichi nuclear plant.

TEPCO said Monday it believed about 300 litres of waste water have escaped and run into a nearby gutter that leads to the ocean before crews could contain the leaks.

The water leaked to the sea is believed to contain 26 billion becquerels of radioactive materials, TEPCO said.

The company said, however, human health should not be affected even after eating sea food caught in the area for every day for one year.

"We again sincerely apologise for causing worries and troubles to the area residents as well as the society at large for releasing water containing radioactive materials," TEPCO said in a statement.

In the weeks after the March 11 earthquake and tsunami hit the plant, TEPCO dumped 10,000 tonnes of lower-level radioactive water into the Pacific Ocean.

Subsequent reports have found the radiation was widely dispersed and did not pose a threat to human or animal life.

Fukushima's makeshift water-treatment system has been hit by a series of problems which forced officials to temporarily shut it down.

But TEPCO said the leak would not hinder its plans to bring the reactors to a state of cold shutdown by the end of the year.

Large areas around the Fukushima plant have been left contaminated with radiation after a series of meltdowns in the world's worst nuclear accident since Chernobyl.

The accident has not directly claimed any lives, but has left tens of thousands of people displaced and rendered whole towns uninhabitable, possibly for decades.