As much as 120 tons of radioactive water may have leaked from a storage tank at Japan's crippled Fukushima nuclear plant, contaminating the surrounding ground, Tokyo Electric Power Company said on Saturday.

The power company has yet to discover the cause of the leak, detected on one of seven tanks that store water used to cool the plants reactors, a spokesman for the company, Masayuki Ono, said at a press briefing.

The company plans to pump 13,000 cubic meters of water remaining in the tank to other vessels over the next two weeks.

Water from the leaking tank, which located 800 meters from the coast, is not expected to reach the sea, Kyodo news wire reported, earlier, citing unidentified officials from the utility.

The company did not say how long the tank had been leaking.

The operator of the Fukushima Daiichi plant has faced a range of problems with controlling ground water and maintaining the massive cooling system built to keep the reactors stable.

The power company said on Friday said it lost the ability to cool radioactive fuel rods in one of the plant's reactors for about three hours. It was the second failure of the system to circulate seawater to cool spent fuel rods at the plant in the past three weeks.

The facility was the site of the worst nuclear disaster since Chernobyl in March 2011 when a magnitude 9.0 earthquake triggered a tsunami that destroyed back-up generators and disabled its cooling system. Three of the reactors melted down.

The storage tanks, pits excavated at the site in the wake of the disaster, are lined with water proof sheets meant to keep the contaminated water from leaking into the soil.

Work to decommission the plant is projected to take decades to complete. – Reuters