Lee Jong-wook, a South Korean doctor, delivered his stark warning as the United States worked to rally states behind a new US plan to fight the disease, which has already killed more than 60 people in Asia and spread to Russia and Europe.
"Human influenza is coming, we know that, and no government, no leaders can afford to be caught off-guard," Lee said.
"We must pounce on human pandemic outbreaks with all medicines at our disposal and at the earliest possible moment," he told a news conference in New York.
"When the pandemic starts, it is simply too late."
US President George W. Bush unveiled a plan at the United Nations on Wednesday under which countries and international agencies would pool resources and expertise to fight bird flu.
His International Partnership on Avian and Pandemic Influenza reflects growing concern that avian flu could becomes a human pandemic, a threat Bush said the world must not allow.
WORSE RISK THAN HIV
Andrew Natsios, head of the US Agency for International Development, said the risk of bird flu was even worse than HIV/AIDS. He urged nations to cooperate fully and not to hide knowledge of the disease when it struck.
"The consequences for the global economy could be massive," Natsios told a small group of reporters. "Without international and national responses we will not stop the disease."
Most of the people killed in Asia since 2003 caught the virus from infected birds. Health experts say the greatest worry is that the highly pathogenic strain of the disease known as H5N1 could mutate and become transmissible between people.
Lee said H5N1 "will acquire this capability -- it's just an issue of timing." Countries far from heavily hit Southeast Asian states would not be safe because the disease was spreading through migratory wildfowl, Lee added.
He urged states like Japan, Switzerland and France with stockpiles of anti-flu drugs to make medicines available for international emergencies.
Paula Dobriansky, US Under Secretary of State for Democracy and Global Affairs, said the United States would convene a senior officials meeting in Washington soon to coordinate policy. Canada will host a global health ministers in the coming weeks to support the US initiative, she said.
Partner countries and agencies include Argentina, Australia, Britain, Canada, China, Japan, Malaysia, Singapore and Russia, as well as WHO, the UN Food and Agriculture Organization and UNICEF, Dobriansky said.
(Additional reporting by Sue Pleming)