"We are looking into the cause and the situation and we'll make that public when we have further information," Edano said.

"At present, we think 10 km evacuation is appropriate."

Japan earlier in the day warned of a meltdown at a reactor at the plant, damaged when a massive earthquake and tsunami struck the northeast coast, but said the risk of radiation contamination was small.

Edano also said that Japan has expanded the evacuation area around the Fukushima Daini nuclear plant. (Reporting by Kiyoshi Takenaka; Editing by Joseph Radford)