Radioactive underwater concentrations in No. 2 reactor at the crippled Fukushima Daiichi nuclear power plant have increased 17 times in one week, the Japanese nuclear safety agency said on Friday.

Experts believe the reason is an ongoing leak of contaminated water from the turbine room of the crippled reactor.

A total of around 60,000 tons of contaminated water is believed to be flooding the basements of the Nos. 1 to 3 reactor turbine buildings as well as trenches connected to them, and the water is hampering work to restore the cooling functions of the reactors lost since the March 11 earthquake and ensuing tsunami.

The plant operator continues to grapple with pools of highly radioactive water found on the plant's premises, with the level of polluted water filling an underground trench edging up again after the company finished pumping out around 660 tons of water.