Some radioactive water may have leaked into the ground from a storage tank at the crippled Fukushima Daiichi nuclear power plant, its operator said Sunday, the latest in a series of troubles at the facility.

The fresh leak comes a day after Tokyo Electric Power Co. (TEPCO) said up to 120 tonnes of contaminated water may have escaped from another of the seven underground reservoir tanks at the tsunami-damaged plant.

But it said the contaminated water was unlikely to flow into the sea.

The tanks store water used to cool down the reactors after radioactive caesium is removed but other radioactive substances remain.

TEPCO said radioactivity was detected in water outside a tank in the latest leak.

"We have determined that a minimal amount of water was feared to have leaked from the tank although there was no decline in the level of water inside the tank," it said in a statement

The series of leakages came after one of the systems keeping spent atomic fuel cool at the plant temporarily failed on Friday, the second outage in a matter of weeks, underlining the precarious fix at the plant.

Nuclear fuel, even after use, has to be kept cool to prevent it from overheating and beginning a self-sustaining atomic reaction that could lead to meltdown.

The plant was hit by the giant tsunami of March 2011 as reactors went into meltdown and spewed radiation over a wide area, forcing tens of thousands of people from their homes and polluting farmland.