Japan's Nuclear and Industrial Safety Agency on Friday raised the nuclear accident seriousness level at Fukushima No.1 nuclear power plant from four to five on an international scale of zero to seven.

The radioactive leaks at the troubled plant have thus been the severest nuclear accident in Japan.

At a press conference, agency officials said the radioactive severity at No. 1, No. 2 and No. 3 reactors was rated at level five while that at No. 4 was rated at level three.

On Sunday the agency rated the incident at the No.1 reactor at level four after radioactive substances were detected there.

The upgrade has made Japan's nuclear leak severity rise to the same level as the Three Mile Island accident in the United States in 1979.