Margaret Chan also said that she backed more equitable access to a future vaccine and urged health officials to resolve their differences on the issue.

Indonesia, the nation worst hit by bird flu with 91 human deaths, has held back most of its virus samples and is demanding guarantees that poor countries get access to affordable vaccines derived from them.

"The sharing of currently circulating viruses is the only way to monitor the emergence of drug-resistant strains. But above all, the sharing of viruses is the foundation of risk assessment," Chan said, opening a four-day WHO meeting.

"The analysis and comparison of viruses give us the first clues, the first early warning that the virus may be evolving in a dangerous way. This is our key difference from the past -- this capacity to pick up the signal that tells us to gear up our defences and prepare our populations," she added.

Jakarta has shared just two specimens this year, both from Indonesian women who died in the tourist resort of Bali in August, according to WHO's top bird flu official David Heymann.

China shared viruses from all of the human cases it had reported this year, he said.

Noting it was the fifth WHO meeting this year on the issue of virus-sharing and vaccines, Chan called for its 191 member states to devise paths towards progress as quickly as possible.

The H5N1 virus has killed 206 of 335 people infected since 2003 in 12 countries, according to the WHO.

Experts fear the constantly mutating virus could change into a form easily transmitted from person to person and sweep the world in months. A pandemic could kill millions of people, shut down businesses and overwhelm health care systems, they warn.

Countries must brace for a pandemic, during which up to 25 percent of workforce could be ill at any given time, Chan said.

This could cause a "meltdown of basic municipal services and slowdown of economic activity", she added.

Sixteen companies are at various stages of licensing a vaccine against H5N1, the virus most experts suspect could spark a pandemic.