Scientists and negotiators at UN talks voiced outrage Monday over the theft of email exchanges between climate scientists, denouncing the hack as an attempt to muddy public opinion on global warming.

Saudi Arabia, meanwhile, called for a neutral investigation into allegations that scientific data had been manipulated to exaggerate the threat of climate change.

At ceremonies opening the 12-day negotiations, the head of the UN's Nobel-winning panel of scientists said he suspected the so-called Climategate affair was a covert attempt to tarnish his organisation.

"Given the wide-ranging nature of change that is likely to be taken in hand, some naturally find it inconvenient to accept its inevitability," said Rajendra Pachauri, chairman of the Intergovernmental Panel on Climate Change (IPCC).

"The recent incident of stealing the emails of scientists at the University of East Anglia shows that some would go to the extent of carrying out illegal acts, perhaps in an attempt to discredit the IPCC."

Pachauri proudly defended the IPCC's reputation as an arena for weighing scientific evidence fairly, neutrally and objectively.

UN climate chief Yvo de Boer also backed the panel's work, built on the consensus of more than 2,500 top scientists.

"I think the IPCC is about the most thorough scientific process that there is," he told journalists in Copenhagen.

The emails, hacked last month, have been seized on by climate sceptics as evidence that scientists twisted data in order to dramatise global warming.

Some of the thousands of messages, purloined from scientists at Britain's University of East Anglia, a top centre for climate research, expressed frustration at the scientists' inability to explain what they described as a temporary slowdown in warming.

They also discussed ways to counter the campaigns of climate naysayers.

The affair, dubbed "Climategate," has provoked a firestorm of controversy, especially in the United States.

"It has sucked up all the oxygen," said James Overland, an Arctic specialist with the National Oceanic and Atmospheric Administration (NOAA), in Copenhagen to open a US-sponsored science centre.

"It just happens to be the topic of the moment," Jonathan Pershing, deputy head of the US delegation at the December 7-18 UN talks, told AFP.

"It is a misrepresentation of the robustness of science. What we end with is that our understanding of the issues is not different... I look at this and I think to myself, it's opportunistic."

Major oil exporter Saudi Arabia, though, said the affair had rattled confidence in climate science.

"The level of trust is definitely shaken, especially now that we are about to conclude an agreement that ... is going to mean sacrifices for our economies," said Mohammed al-Sabban, the kingdom's top climate negotiator.

Sabban called for an "independent" international investigation, saying the IPCC was unqualified to carry it out.

The panel co-won the 2007 Nobel Peace prize for its series of massive reports on global warming.

Its landmark publication, the Fourth Assessment Report, declared that the evidence of warming was "unequivocal" and damage to glaciers, snowfall and changing seasons were among the signs that climate change was already on the march.

In remarks to AFP, Pachauri said he did not believe that the affair would sway the opinion of people who had carefully weighed the evidence for or against climate change.

"I think people are informed enough to realise that the Fourth Assessment Report is completely objective, totally unbiased and solid in its scientific assessment.

"The only debate is who is behind it, I think we should catch the culprits."

The American Association for the Advancement of Science (AAAS) stood by the consensus among mainstream scientists.

"The vast preponderance of evidence, based on years of research conducted by a wide array of different investigators at many institutions, clearly indicates that global climate change is real, it is caused largely by human activities, and the need to take action is urgent," said AAAS executive officer Alan Leshner.