A top U.N. health official has been named to organize a campaign aimed at preventing a worldwide avian flu pandemic in humans. The official has repeated a warning from scientists that the flu virus could mutate and spread across the globe.

Dr. David Nabarro of the World Health Organization (WHO) says bird flu could spread to humans at any time, killing anywhere from five million to 150 million people.

Dr. Nabarro is one of WHO's top public health experts.  He was named Wednesday to lead a global effort to head off an outbreak of the virus, which has killed more than 60 people in four Asian nations in the past two years.

Until now, most of the victims have been people in close contact with infected birds. But Dr. Nabarro says there is a high probability that the virus will mutate and quickly spread through human-to-human contact.

"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's currently causing bird flu in Asia, the short form is H5N1, which has already resulted in millions of poultry deaths and billions of dollars of economic loss," he said.

Dr. Nabarro's appointment came after a group of Asian leaders pressed Secretary-General Kofi Annan at a recent summit to coordinate a response to a feared flu pandemic. Dr. Nabarro said the concerns were triggered by news that the virus has spread through flocks of migratory birds.

"We found the bird flu virus H5N1 is now also in migrating geese, ducks and other wild fowl and probably moving across the world as flocks migrate beyond the area of the center of current avian flu epidemic," he added.

Dr. Nabarro noted that a flu epidemic in 1918 killed more than 40 million people. Other epidemics in 1957 and 1968 were less deadly, but caused greater social and economic disruption.

He said he is almost certain there will be another pandemic soon, and said the key to preventing what he called a "nightmare scenario" is preparedness.

"Preparing so that if there is a species jump, if the virus moves and mutates from the bird population to the human population, we are ready," he said.  "Most scientists reckon there will be a very short interval between the discovery of the mutant virus that causes the next influenza pandemic and the time when that pandemic really starts to get out of control.  It will be a matter of weeks."

Dr. Nabarro says he is greatly worried about a bird flu outbreak in a country where health systems are limited, or where international assistance may not be welcome.

Several countries, however, have already joined forces to coordinate their preparations. The United States announced a new International Partnership on Avian and Pandemic Influenza this month at the U.N. World Summit.  A planning meeting of the group will be held next week in Washington.