Four more children are suspected of suffering from thyroid cancer in the latest survey on the possible health impact of the 2011 triple meltdown at Tepco's wrecked Fukushima No. 1 plant, sources said Tuesday.

The four, who were 6 to 17 years old at the time of the disaster, had been diagnosed as not having the cancer in the first survey that was conducted within three years of the meltdowns, they said.

The first survey covered all 370,000 children in the prefecture who were aged 18 or younger at the time of the disaster. The second survey , which began last April, covers some 385,000 children, adding those born a year after the disaster struck.

Researchers at Fukushima Medical University, which has been leading the study, will work to confirm if the four have developed the cancer and carefully study if the cases are due to the influence of radiation, according to the sources.

The university takes the results of the first survey as the basic data in assessing whether cases of the cancer may increase in the future.

The Fukushima Prefectural Government said in August that 57 children in the first survey had been confirmed as suffering from thyroid cancer and 24 others were suspected of having it.

The four lived in the municipalities of Okuma, Fukushima, Date and Tamura at the time.

External radioactive exposure levels in three of the four municipalities were estimated to be up to 2.1 millisieverts within four months of March 11, 2011, the day of the Great East Japan Earthquake and tsunami that triggered the nuclear meltdowns, the sources said.