Japan's nuclear regulator on Wednesday upgraded its evaluation of a radioactive water leak at the crippled nuclear plant in Fukushima to a level three "serious incident".

The assessment, on an international scale of zero to seven with seven being the worst, came after operator Tokyo Electric Power Company (TEPCO) said some 300 tonnes of radioactive water was believed to have leaked from a tank at the Fukushima Daiichi nuclear power plant.

It was the worst such leak since the crisis began in March 2011 when a quake-generated tsunami knocked out reactor cooling systems and sparked meltdowns.

Japan's Nuclear Regulation Authority raised the evaluation from level one, which means "anomaly" on the UN's International Nuclear Event Scale (INES).

Level three cases on the INES scale are described as "serious incidents" with "exposure in excess of ten times the statutory annual limit for workers".

The nuclear crisis at Fukushima two years ago is one of only two events classified as level seven -- the other being the Chernobyl disaster.