Japan's 2011 Fukushima disaster raised the cancer risk for people living near the atomic plant, but no jump in cases is expected elsewhere, the UN's health agency said Thursday, sparking an angry reaction from anti-nuclear campaigners.

Within a 20-kilometre (12-mile) perimeter of the plant, rates of thyroid cancer among women who were exposed to radiation as infants were expected to be up to 1.25 percent, the World Health Organisation said in a report.

This represented a 70-percent increase over the baseline risk of thyroid cancer over a Japanese woman's lifetime, which is 0.75 percent, the UN health agency noted.

"In view of the estimated exposure levels, an increased risk of cancer was the potential health impact of greatest relevance," Maria Neira, the WHO's director of public health and environment, told reporters as she launched the 166-page report.

"Outside the most exposed areas, so outside of Fukushima prefecture, and even in some areas of Fukushima prefecture, the predicted risk remains low and even non-observable. That means we didn't observe any increase in cancer above what we call the natural variation in baseline rates," she explained.

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 months that followed.

Radioactive iodine released in nuclear accidents tends to accumulate in thyroid glands.

In the 1986 Chernobyl disaster in what was then Soviet Ukraine, a noticeable increase in thyroid cancer cases was detected among children in the affected area.

The WHO produced its Fukushima report using data from Chernobyl, the 1945 US atomic bombings of Hiroshima and Nagasaki in Japan, other research and complex mathematical modelling.

Senior WHO official Angelika Tritscher said the agency had used thyroid cancer as a benchmark because it is one of the forms of the disease easiest to monitor, and focused on women because of gender differences in cancer sensitivity.

Other forms of cancer also looked set to rise, albeit to a lesser extent, the agency said.

It pointed to a slightly higher risk of breast cancer among women exposed as infants, and of leukaemia among men.

"The purpose of this report is to give an indication of the magnitude of the risk, it's not absolute numbers. It's probability," said Tritscher.

Radiation doses from the stricken plant were not expected to cause an increase in miscarriages, stillbirths and physical and mental conditions that could affect babies born after the accident, the WHO said.

The agency's message was more positive still for Japan's neighbours.

"In neighbouring countries and the rest of the world, the estimated increase in cancer risk is negligible. So there's no additional health risk expected due to the Fukushima accident," said Tritscher.

Anti-nuclear campaigners slammed the report.

"The WHOs flawed report leaves its job half done," said Rianne Teule, Greenpeace International's nuclear radiation expert.

In a statement, Greenpeace charged that the WHO failed properly to analyse the impact of radiation on people inside the 20-kilometre evacuation zone who were not able to leave the area quickly.

"The WHO report is clearly a political statement to protect the nuclear industry and not a scientific one with peoples health in mind," said Teule.

"The WHO and other organisations must stop downplaying and hiding the impact of the Fukushima disaster and call for more emphasis on protecting the millions of people still living in contaminated areas," she insisted.

The WHO said in-depth analysis of the health impact of the disaster would be crucial for many years.

"The WHO report underlines the need for long-term monitoring of those who are at high risk, along with the provision of necessary medical follow-up and support services," said Neira.