Radioactive substance about 10,000 times the limit was found at the groundwater around the No. 1 reactor of the troubled Fukushima No.1 nuclear power plant, local media reported Friday.

A official with the plant's operator Tokyo Electric Power Co. said Thursday that the radiation level is "extremely high," according to Kyodo News report.

Also on Thursday, the level of radioactive iodine-131 found in seawater near the nuclear power plant, about 240 kilometers northeast from Tokyo, has soared to its highest reading yet at a concentration of 4,385 times the legal limit.

The Japanese authorities have been stepping up efforts to fix the plant crippled by devastating March 11 earthquake and tsunami.