This contaminated groundwater has breached an underground barrier, is rising toward the surface and is exceeding legal limits of radioactive discharge, Shinji Kinjo, head of a Nuclear Regulatory Authority (NRA) task force, told Reuters.

Countermeasures planned by Tokyo Electric Power Co are only a temporary solution, he said. 

Tepco's "sense of crisis is weak," Kinjo said. "This is why you can't just leave it up to Tepco alone" to grapple with the ongoing disaster.

"Right now, we have an emergency," he said. 

Tepco has been widely castigated for its failure to prepare for the massive 2011 tsunami and earthquake that devastated its Fukushima plant and lambasted for its inept response to the reactor meltdowns. It has also been accused of covering up shortcomings.