A flu pandemic could happen at any time and kill between 5-150 million people, a UN health official has warned.
Experts fear birds will carry the virus across borders
David Nabarro, who is charged with co-ordinating responses to bird flu, said a mutation of the virus affecting Asia could trigger new outbreaks.
"It's like a combination of global warming and HIV/Aids 10 times faster than it's running at the moment," Dr Nabarro told the BBC.
But the World Health Organisation has distanced itself from the figure.
The WHO spokesman on influenza, Dick Thompson, told a news conference in Geneva that the WHO's official estimate of the number of people who could die was between two million and 7.4 million.
"There is obvious confusion, and I think that has to be straightened out. I don't think you will hear Dr Nabarro say the same sort of thing again," Mr Thompson said.
Bird flu has swept through poultry and wild birds in Asia since 2003.
It has killed huge numbers of birds and led to more than 60 human deaths.
Prepared for worst
"The range of deaths could be anything between 5m and 150m," the UN's new co-ordinator for avian and human influenza said in his BBC interview.
Dr Nabarro said he stood by the figure drawn from the work of epidemiologists around the world.
"My reason for giving the higher figure is simply that I want to be sure that when this next flu pandemic does come along, that we are prepared for the worst as well as for the mildest," he said.
In an earlier interview with the BBC, he said the likelihood that the Asian virus could mutate and jump to humans was high.
"The consequences in terms of human life when the pandemic does start are going to be extraordinary and very damaging," he said.
Because it has moved to wild migratory birds there is a possibility "that the first outbreak could happen even in Africa or in the Middle East", he warned.
The comments came as agriculture ministers from the Association of South East Asian Nations (Asean) endorsed a three-year plan to combat the spread of the virus, and pledged $2m to fund research and training.
Dr Nabarro said the number of deaths from any future influenza pandemic would depend on where it started, how quickly it was discovered and the kind of response they got from governments.
H5N1 BIRD FLU VIRUS
Principally an avian disease, first seen in humans in Hong Kong, 1997
Almost all human cases thought to be contracted from birds
Isolated cases of human-to-human transmission in Hong Kong and Vietnam, but none confirmed
"I believe that the work we're doing over the next few months will make the difference between, for example, whether the next pandemic leads us in the direction of 150 or in the direction of five."
The appointment of Dr Nabarro is an indication of how seriously the UN is taking the threat, the BBC's UN correspondent Suzannah Price says.
In his new role, he is meant to ensure the UN has a co-ordinated response to bird flu and that it helps global efforts to prepare for any human flu pandemic, our correspondent says.