"We have been experiencing a relentless spread of avian flu" among migratory birds and domestic poultry, Lee Jong-wook, director-general of the World Health Organization told a meeting of 600 health experts and planners, the first attempt to devise a global strategy in case the bird flu virus changes to transmit easily among humans.
Lee stressed that a human flu pandemic has yet to begin anywhere in the world.
"However, the signs are clear that is coming," he said, noting that a changed avian flu virus caused the deadly "Spanish" flu pandemic that killed tens of millions of people in 1918-1919.
Already the virulent H5N1 strain of avian flu, which appeared in Hong Kong in 1997, is killing birds in 15 countries of Europe and Asia, he said.
"It is only a matter of time before an avian flu virus -- most likely H5N1 -- acquires the ability to be transmitted from human to human, sparking the outbreak of human pandemic influenza," Lee said.