The UN report found that 56 people have died so far and 4,000 may eventually perish from the effects from the 1986 disaster at the Ukrainian nuclear power plant -- many fewer than initially feared.

But Bellona, which specializes in Russian nuclear issues, said the UN figures underestimated the reality.

The potential death toll of 3,940 is "just a small fraction" of the total, Bellona nuclear physicist Nils Boehmer told AFP, adding that "many questions remain unanswered".

He suggested that the figures, which were provided by Ukrainian authorities, were skewed.

"It's easy to get the figures that you want. ... I'm a little sceptical as to the nature of the information that we're getting," he said.

According to the 600-page document, more than 600,000 people suffered radiation exposure from the accident, including reactor staff, emergency and recovery personnel and residents of surrounding areas.

The report was presented at a conference Tuesday in Vienna joining nuclear, health and development experts from eight UN agencies, meeting under the aegis of the International Atomic Energy Agency.

"The goal of the International Atomic Energy Agency is to promote nuclear power for civilian uses. It has a vested interest in minimizing the impact," Boehmer stressed.

"Before, it was talking of fewer than 100 victims. Now it's talking about several thousand dead. This is a dramatic increase in the death toll," he said.

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