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, head of the World Health Organization (WHO) said Monday.

"We don't know when this will happen. But we do know that it will happen," WHO Director-General Lee Jong-wook said at the opening ceremony of a three-day global meeting on bird flu and human pandemic flu held in Geneva.

Lee said this is the time to build global consensus, and this is the time for every country to prepare their national action plan -- and act on it.

"If we are unprepared, the next pandemic will cause incalculable human misery. Both directly from the loss of human life, and indirectly through its widespread impact on security. No society would be exempt. No economy would be left unscathed," he warned.

He highlighted the issues top on the agenda of the meeting, which is co-organized by WHO, the Food and Agriculture Organization, the World Organization for Animal Health and the World Bank.

The issues include how to prevent and contain the spread of H5N1 virus among birds and from birds to humans, how to increase country capacity in surveillance, early detection, diagnosis and reporting of cases -- both animal and human.

"We must decide how to approach policies on research, development and production of vaccines and antivirals, reviewing manufacturing capacity, and access issues... We must decide how best to communicate both the risks and the positive areas for action by all communities, including strategies for business and societal continuity," he added.

More than 400 animal and human health experts, senior policy makers, economists and industry representatives participated in the meeting to work toward a global consensus to control the H5N1 bird flu virus in domestic animals and prepare for a potential human flu pandemic.