A Japanese government-backed researcher said Friday no health effects from radiation released by the stricken Fukushima nuclear plant have been seen in people living nearby.

The pronouncement by Kazuo Sakai of Japan's National Institute of Radiological Sciences is the latest by authorities seeking to quell fears over the long-term effects of the disaster.

But it was dismissed by campaign group Greenpeace who said the government should not seek to play down health worries.

"Since the accident in Fukushima, no health effects from radiation have been observed, although we have heard reports some people fell ill due to stress from living as evacuees and due to worries and fears about radiation," Sakai said.

"We know from epidemiological surveys among atomic-bomb victims in Hiroshima and Nagasaki that if exposure to radiation surpasses 100 millisieverts, the risk of cancer will gradually rise.

"To put it the other way round, we can't say risk of cancer will rise if you are exposed to radiation lower than 100 millisieverts," he said, adding that most people measured had radiation exposure of 20 millisieverts or less.

Sakai said radiation is not at "the level we have to worry about its health effect," for people in Fukushima, taking into account exposure from the atmosphere and ingestion from food.

His comments came as the Fukushima prefectural government panel said this week three people who were 18 or younger when the nuclear crisis erupted in March 2011 have been diagnosed with thyroid cancer.

Radioactive iodine released in nuclear accidents tends to accumulate in thyroid glands, particularly in young people. In the 1986 Chernobyl disaster, a noticeable increase in thyroid cancer cases was detected among children in the affected area.

Referring to the thyroid cancers reported in Fukushima, Sakai said "there is no clear link between the cancers and exposure to radiation, as empirical knowledge says it takes several years before thyroid cancer is detected after exposure to radiation."

"It is important, however, to monitor these cases," he added, noting that comparison with the pre-accident situation and other regions was necessary.

Kazue Suzuki, nuclear campaigner at Greenpeace, who is not a scientist, said Japan should not try to play down the potential dangers.

"Japan should pour more energy into prevention of diseases including thyroid cancer than talking down the risk of low-level radiation."

"Even if there is no comparative epidemiological data, the government should err on the side of caution and carry out more frequent health checks among residents not only in Fukushima but in other prefectures," she said.

A massive undersea earthquake in March 2011 sent a huge tsunami crashing into Japan's northeast, crushing whole communities and sending nuclear reactors on the coast into meltdown.

Around 19,000 people were killed by the natural disaster, but no one is officially recorded as having died as a direct result of the radiation that spewed from the crippled units in the following months.