Dr David Nabarro of the World Health Organisation called on governments to take immediate steps to address the threat at a news conference following his appointment as the new UN coordinator to lead a global drive to counter a human flu pandemic.
"We expect the next influenza pandemic to come at any time now, and it's likely to be caused by a mutant of the virus that is currently causing bird flu in Asia," he said.
The H5N1 strain of bird flu has swept through poultry populations in Asia since 2003, infecting humans and killing at least 65 people, mostly poultry workers, and resulting in the deaths of tens of millions of birds.
The virus does not pass from person to person easily, but experts believe this could change if the virus mutates.
Nabarro said with the almost certainty of another influenza pandemic soon, and with experts saying there is a high likelihood of the H5N1 virus mutating, it would be "extremely wrong" to ignore the serious possibility of a global outbreak.
"The avian flu epidemic has to be controlled if we are to prevent a human influenza pandemic," Nabarro said.
"The avian flu epidemic has to be controlled if we are to prevent a human influenza pandemic"
Dr David Nabarro,
World Health Organisation
The 1918 influenza pandemic killed more than 40 million people, and there were subsequent pandemics in 1957 and 1968 that had lower death rates but caused great disruption, he said.
In a new pandemic, Nabarro said, "the range of deaths could be anything between 5 and 150 million".
"The work we're doing over the next few months on prevention and preparedness will make the difference between, for example, whether the next pandemic leads us in the direction of 150 (million) or in the direction of 5 (million)," he said.
Nabarro said Asian leaders met with Secretary-General Kofi Annan during the recent UN summit and asked for UN assistance in coordinating the response to the bird flu epidemic.
Annan asked him to take a leave from his current post as WHO's executive director for sustainable development and health environments to become the UN system's coordinator for avian and human influenza.
Nabarro said he plans to travel to Washington on Friday to work with the US State Department on preparations for the first meeting of the International Partnership on Avian and Pandemic Influenza on 7 October.
Asian leaders asked Annan for
UN assistance in tackling bird flu
The US initiative, announced by President George Bush at the UN summit on 14 September, is designed to increase global readiness to deal with a human flu pandemic and will "garner political will", he said.
In Jakarta on Friday, Indonesian foreign affairs spokesman Yuri Thamrin said: "Bird flu is such a dangerous disease that can easily transcend national borders through animal and human migration.
"There should be international cooperation."
Asean said it will endorse a global plan to contain avian influenza, which has killed 66 people in four Asian countries since late 2003 and led to an estimated $15 billion in losses for the poultry trade.
"The creation of the animal health trust fund has been approved," Philippine Agriculture Secretary Domingo Panganiban told Reuters by telephone from Tagaytay, a resort city south of Manila where Asean agriculture ministers were meeting on Friday.
Since 2003 at least 66 Asians
have died of avian flu
The fund would also be used to prevent the spread of other illnesses affecting animals such as foot-and-mouth disease and hog cholera, other officials have said.
Philippine Agriculture Undersecretary Segfredo Serrano said Asean would also endorse the global bird flu plan proposed by the world animal health body OIE, the Food and Agriculture Organisation (FAO) and the WHO.
The three agencies plan to hold a bird flu conference in December to try to raise the $102 million they say is needed over the next three years to contain the virus.
The proposed programme needed the endorsement of Asean to give it credibility among donor countries and multilateral agencies, Subhash Morzaria, chief technical adviser of the FAO in Asia and the Pacific, said on Thursday.
The money will be used to help affected countries and fund research, training of personnel and campaigns against bird flu.