The utility said new hot spots were found in the compound where radioactive water from the plant is stored, and radiation levels in other areas increased to highly dangerous levels.
The discoveries indicated possible leaks in the tanks, which were hurriedly installed to store radioactive water in the aftermath of the 2011 earthquake and tsunami that damaged the Fukushima plant, The Wall Street Journal said Sunday.
Tepco increased its monitoring of the four huge tanks last month after it was discovered that 79,000 gallons of contaminated water had leaked from one of them.
Massive volumes of water had been pumped into the plant to cool its reactors after the quake. But the water picked up radioactive materials in the process and is too dangerous to be released back into the sea.