A United Nations report said Monday that the number of people killed in almost 20 years since the world's worst nuclear accident is 56 - much lower than previously estimated.

U.N. officials said the death toll was 47 emergency workers and nine children who had died of thyroid cancer.

KIEV, UKRAINE: A Ukrainian woman cries by a gravestone at the Memorial of Chernobyl victims during a ceremony in Kiev 26 April 2005. (SERGEI SUPINSKY/AFP/Getty Images)

The report -- compiled by the Chernobyl Forum, which includes eight U.N. agencies -- said the final death toll was expected to reach about 4,000, and that the greatest damage to human health was psychological.

Most of the 4,000 expected deaths would be among emergency workers exposed to high radiation doses shortly after the accident. They were at higher risk of contracting cancer decades later.

The disaster occurred at 1:24 a.m. on April 26, 1986, when an explosion at Reactor 4 of the Ukrainian power plant spewed a cloud of radioactivity over Europe and the Soviet Union, particularly contaminating Belarus, Russia and Ukraine.

"Claims have been made that tens or even hundreds of thousands of persons have died as a result of the accident. These claims are exaggerated," the Chernobyl Forum report said.

"We have not found profound negative health impacts to the rest of the population in surrounding areas, nor have we found widespread contamination that would continue to pose a substantial threat to human health, with a few exceptional, restricted areas," said the forum's Chairman, Burton Bennett.

About 4,000 people developed thyroid cancer as a result of the accident, most of them children and adolescents in 1986.

The report said the survival rate had been almost 99 percent, based on figures in Belarus.

Apart from reactor staff and emergency workers exposed on the day of the disaster, most emergency workers and residents of contaminated areas received relatively low radiation doses, comparable to background levels, the U.N. said in a statement.

Apart from thyroid cancer, there was no evidence of any increase in cancer or leukemia rates among local residents, it said, nor was there evidence of decreased fertility or of a higher rate of congenital malformations.

For the 350,000 people moved out of contaminated areas, however, relocation was a "deeply traumatic experience" which often left them unemployed, the U.N. statement said.

People from areas near Chernobyl were labelled as 'victims' rather than 'survivors', which led them to view themselves as "helpless, weak and lacking control over their future", it said.

"This, in turn, has led either to over-cautious behaviour and exaggerated health concerns, or to reckless conduct, such as ... overuse of alcohol and tobacco, and unprotected sexual activity," the statement added.

Benefits offered to victims were expanded to 7 million people. The report said these benefits needed to be scaled down or target only high-risk groups.

Many evacuated areas were now safe, the report said. Apart from the still closed, highly contaminated 30-km area surrounding the reactor and some closed lakes and restricted forests, radiation levels had mostly returned to acceptable levels, the statement said.

The forum's report "Chernobyl's Legacy: Health, Environmental and Socio-Economic Impacts" examines the effects of the disaster as the 20th anniversary approaches.

The environmental organization, Greenpeace, challenged the findings, saying in a statement that the reports it was based on contained contradictory information and research had been omitted.

The Chernobyl Forum includes the International Atomic Energy Agency (IAEA), World Health Organization (WHO), the World Bank and the governments of Belarus, Russia and Ukraine.