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.
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Bird flu
"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.
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
In pictures: Indonesia alert Q&A;: Avian flu
"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.
A United Nations health expert who is co-ordinating the response to the recent south-east Asian bird flu outbreak, has warned that the disease could kill 'up to 150 million' people in the near future.

Dr David Nabarro told the BBC that a new outbreak of bird flu was possible, with migratory birds carrying the disease to Africa or the Middle East where the human death toll "could be anything between 5m and 150m".

"The consequences in terms of human life when the pandemic does start are going to be extraordinary and very damaging."

Bird flu — also known as avian influenza — is a type of influenza virulent in birds. It was first identified in Italy in the early 1900s and is now known to exist worldwide. It was not known to affect humans until the first recorded case in Hong Kong in 1997.

Bird flu has spread rapidly through poultry and wild birds in Asia since 2003, killing huge numbers of birds and more than 60 humans.

"It's like a combination of global warming and HIV/Aids 10 times faster than it's running at the moment," Dr Nabarro said.

In contrast to the shock-factor of Dr Nabarro's comments, the U.N. has distanced themselves from his remarks.  The World Health Organization's official estimate for the death toll in the event of a mutation of the H5N1 virus is between 2 and 7.4 million.  WHO influenza spokesman, Dick Thompson, implied that Dr Nabarro would be taken to task over his remarks, stating, "I don't think you will hear Dr Nabarro say the same sort of thing again."