Plutonium finds in soil at the plant have raised public alarm over the world's worst nuclear accident since Chernobyl in 1986, which has overshadowed the humanitarian disaster triggered by an earthquake and tsunami on March 11 that left 27,500 people dead or missing.

France and the United States will help Japan in its battle to contain radiation but authorities said they still had no idea when they would be able to stabilize the facility, 150 miles north of Tokyo.

"We are not in a situation where we can say we will have this under control by a certain period," Chief Cabinet Secretary Yukio Edano told a briefing.

New readings showed a spike in radioactive iodine in the sea off the plant to 3,355 times the legal limit, the state nuclear safety agency said, although it played down the impact, saying people had left the area and fishing had stopped.

Pollution of the ocean is a concern for a country where fish is central to the diet.

Reuters