Up to 120 tonnes of radioactive water may have "escaped" from an underground storage tank at the Fukushima Daiichi nuclear power plant, an official announced Saturday.

"We are transferring the remaining water from the tank to others," said a Tokyo Electric Power Co (TEPCO) spokesman. The company claims that it is "unlikely" any of the contaminated water found its way into the ocean. 

The contaminated water may have leaked from one of the seven underground reservoir tanks which stores water previously used to cool down the nuclear reactors, AFP reports.

The news follows reports Friday that one of the plant's cooling systems had failed temporarily, the second outage in a matter of weeks.

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