Tokyo authorities said water at a purification plant for the capital of 13 million people had 210 Becquerels of radioactive iodine, more than twice the safety level for infants.

"This is without doubt, an effect of the Fukushima Daiichi plant," a Tokyo metropolitan government official said, in reference to the damaged nuclear plant 250 km (150 miles) north of the city.

Tokyo governor Shintaro Ishihara, however, said the radiation level posed no immediate health risk and water could still be used.

Crystallising international concern, the US Food and Drug Administration said it is stopping imports of milk, vegetable and fruit from four prefectures in Japan's crisis-hit northeast.

The Republic of Korea may be next to ban Japanese food after the world's worst atomic crisis since Chernobyl in 1986. France this week asked the European Commission to look into harmonising controls on radioactivity in imports from Japan.

At the six-reactor Fukushima plant, crippled by a March 11 earthquake and tsunami, engineers are battling to cool reactors to contain further contamination and avert a meltdown.