Japan's science and technology ministry said Saturday that tiny amounts of radioactive iodine have been detected in tap water in Tokyo and prefectures neighboring Fukushima.

The detection came after the twin disasters of the March 11 earthquake and ensuing tsunami caused radioactive leaks at the Fukushima No. 1 nuclear power plant.

According to the ministry, traces of cesium have also been found in tap water in Tochigi and Gunma in addition to the radioactive iodine found in the two prefectures as well as in Niigata, Chiba and Saitama.

But the dose of the radioactive substances poses no threat to human health even if they are taken in.

The Nuclear Safety Commission of Japan limits an intake of iodine at 300 becquerels per kilogram of water and of cesium at 200 becquerels.