Japan's nuclear watchdog on Friday dispatched an inspection team to the crippled Fukushima plant after workers found a huge toxic water leak and unexplained radiation hotspots.

Earlier this week around 300 tonnes of radioactive liquid is believed to have escaped from one of the hundreds of tanks that hold polluted water, some of which was used to cool the broken reactors, in an episode dubbed the most serious in nearly two years.

On Thursday workers looking for other holed tanks found two spots near other containers where radiation was unexpectedly high, although they could see no leaks.

The 15-member team, including Nuclear Regulation Authority committee member Toyoshi Fuketa and experts on radiation and water flow, was due to start the inspection in the morning, an agency official said.

"They plan to inspect areas particularly near where the water was found escaping," he said.

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

The quake and tsunami-sparked 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.

Plant owner Tokyo Electric Power Co (TEPCO) has said puddles of water near the holed tank were so toxic that anyone exposed to them would receive the same amount of radiation in an hour that a nuclear plant worker in Japan is allowed to receive in five years.

The company said the leak may have carried radioactive materials out to sea. Groundwater that has mixed with polluted water has already seeped into the ocean, with TEPCO launching an operation to pump it out of 28 wells, the company said Friday.

More than two years after reactor meltdowns sparked by the quake and tsunami of March 2011, 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, amid fears that the utility -- which has been effectively nationalised -- is not up to the task.