Minute traces of radioactive substances have been detected in tap water in nine prefectures following radioactive leaks from the troubled Fukushima Daiichi nuclear power plant, local media reported Monday, citing government data.

But the amount of the health-threatening substances were below the safety intake limits set by the Nuclear Safety Commission.

According to a nationwide survey by the science and technology ministry, radioactive iodine was detected in tap water of the six prefectures of Saitama, Chiba, Tokyo, Kanagawa, Niigata and Yamanashi while iodine and cesium were detected in Tochigi and Gunma prefectures.

In Fukushima Prefecture, where the nuclear plant is located, iodine was detected in a sample collected at 8 a.m. Monday.

In Tokyo cesium was detected on Saturday but was not found Sunday.

The safety intake limit set by the Nuclear Safety Commission is 300 becquerels per kiloliter of water for iodine and 200 becquerels for cesium.

Radioactive leaks were detected after a series of explosions and fires at four of the plant's six reactors following the failure of their cooling functions due to the damaged power supplies in the 9.0 magnitude earthquake that hit Japan on March 11.

The authorities advised residents living within 20-30 km radius of the plant to stay indoors and set the area 20 km around the plant as an exclusion zone where residents have been evacuated.