The radiation level of the groundwater had increased 10 fold in one week at the troubled Fukushima No. 1 nuclear power plant, Kyodo News reported Thursday, citing operator sources.

Earlier on Thursday, the country's nuclear safety agency said that the level of contaminated water in its underground trench turned out to be rising again.

The operator of the plant, Tokyo Electric Power Co., removed some 660 tons of highly contaminated water in the past two days from one of the trenches to a "condenser" inside the No. 2 reactor turbine building. The condenser has a capacity to store 3,000 tons of liquid.

Radioactive water was found in the basements of the Nos. 1 to 3 reactor turbine buildings, and the nearby trenches connected to them. The water totaled about 60,000 tons.

Removing the water to nearby tanks and other storage places is considered vital to the attempt of restoring the key cooling functions at the reactors. Japan's nuclear safety agency on Tuesday had raised the severity level of the accident at the crippled Fukushima No.1 nuclear power plant to 7, the worst on an international scale, from the current 5.