TOKYO -- Small amounts of radioactive substances have been detected in the breast milk of five women in Japan, online newspaper Japan Today reported Thursday, citing a study by a citizen's group.

Taking samples from 41 women across five prefectures, the tests found cesium in the breast milk of four women in Tokyo, Fukushima and Ibaraki, and radioactive iodine in the breast milk of a woman in Fukushima.

The study, released Wednesday, was conducted one month after a magnitude-9.0 earthquake and tsunami rocked Japan’s northeast coast, triggering a nuclear crisis at the Fukushima nuclear plant which saw radiation leak into the ground, sea and soil.

Reports of contamination in water and food emerged in the weeks following the March 11 twin disasters.

Safety levels of radioactive substances in breast milk have not been set by the Japanese government but readings -- 5.5 becquerels of iodine and up to 10.5 becquerels of cesium -- in all five cases were well below the safe levels -- 100 becquerels of radioactive iodine and 200 becquerels of cesium -- for tap water consumption by infants.

The group has called on health authorities to make testing available to concerned parents.