TOKYO (AFP) - Fukushima operator Tokyo Electric Power Co (Tepco) said on Saturday it would empty two more coolant tanks that hold radioactive water over fears of fresh leaks at the crippled nuclear plant.

Earlier this week, Tepco said around 300 tonnes of radioactive liquid was believed to have escaped from one of the hundreds of tanks used to cool the broken reactors.

The episode was dubbed the most serious since the plant went into meltdown in 2011 after being hit by a quake and tsunami.

Tepco said on Saturday that the affected tank was one of three to have been relocated from their original zone because of ground subsidence in the area.

Tepco has not yet pinpointed the source of the leak in the first tank, but there are fears the relocation may have been connected with the incident.

Accordingly, the company has decided to pump out water from the other two starting on Sunday, a company official said.

Nuclear regulators on Wednesday said the leak represented a level-three "serious incident" on the United Nations' seven-point International Nuclear Event Scale (INES), raising the alert from level one, an "anomaly".

The meltdowns at the plant in March 2011 were ultimately categorised as level seven on the INES scale. The Chernobyl disaster in 1986 is the only other incident to have been given the most-serious ranking.

More than two years after the disaster at Fukushima, Tepco continues to struggle with the clean-up, a project expected to take around four decades.

A catalogue of mishaps, often accompanied by a perceived unwillingness to publicly reveal the extent of problems, is leading to a growing chorus warning of the need for outside experts to step in and take control of the operation.

Critics say the utility - which has been effectively nationalised - is not up to the task.