"I am often asked if the effort invested in pandemic preparedness is a waste of resources," director general Margaret Chan told a regional meeting of the world organisation.
"Has public health cried wolf too often and too loudly?" she said in a speech.
"Not at all. Pandemics are recurring events. We do not know whether the H5N1 (avian influenza) virus will cause the next pandemic. But we do know this: the world will experience another influenza pandemic sooner or later."
WHO regional director Shigeru Omi noted that bird flu deaths in the Western Pacific -- which excludes Indonesia -- had fallen from 19 two years ago to five in the past year.
But he said the virus was still "entrenched" in several countries.
"Because the virus continues to evolve and mutate, we must maintain constant vigilance," he said.
Speed would be the key in handling any human pandemic triggered by bird flu, he said.
"If a human pandemic associated with avian influenza were to break out in the region, rapid containment would be our highest priority. Such an effort would require the massive deployment of antiviral drugs, personal protection equipment and other supplies."
A stockpile was established in Singapore in April with the support of Japan and the Association of Southeast Asian Nations, he noted, urging members to consider using military transport planes to move equipment to affected areas.
Takeshi Kasai, WHO regional adviser for communicable disease surveillance and response, reiterated that the main fear is of H5N1 mutating into a forum easily transmissable between humans.
He told AFP in an interview that past experience and data indicated it might be high time for a new human global influenza outbreak, following pandemics in 1968, 1953 and 1918.
"Sadly, the H5 virus is mutating and changing very rapidly. Usually the bird flu virus changes slowly but this one changes very, very fast," he said.
Kasai said it was unclear if this was an indication that it could mutate into a form easily transmissible between humans. "But these are the facts that make the WHO concerned."
"I'm sure if people are ready, its impact would be low, but if they are not, there would be big disasters."
Omi in his speech noted progress in fighting other regional diseases. He said the Western Pacific had become the only WHO region to meet intermediate 2005 targets for tuberculosis control.
It was also making progress against HIV/AIDS, with prevalance among adults falling in some countries. In Cambodia the percentage had fallen from above two percent in 1998 to around 0.9 percent now.
Deaths from malaria continued to fall but drug-resistant strains hampered control efforts, Omi said. And dengue fever and dengue haemmorhagic fever remained "major public health problems" in many regional countries.
He noted a massive outbreak in Cambodia this year, with more than 30,000 infections and 327 deaths. Malaysia, the Philippines, Singapore and Vietnam also reported increased cases this year.
Omi said the Western Pacific "continues to bear a disporoportionate share of the world's suicide burden." The WHO had begun a project to counter the trend in partnership with the Australian Institute for Suicide Research and Prevention.