Tokyo - Japanese nuclear regulators raised the severity of the latest leak of radiation-contaminated water at a damaged plant to the level of "serious incident" on Wednesday.
Plant operator Tokyo Electric Power Co said last week about 300 tons of highly contaminated water had leaked from a storage tank at the Fukushima Daiichi plant.
The Nuclear Regulation Authority raised the severity of the leak on the International Nuclear Event Scale from level 1 - an "anomaly" - to level 3.
The leak was the worst since the start of the nuclear disaster in 2011, the operator said.
Level 7 - "major accident" - was applied when three of the reactors suffered meltdowns.