WHO chief warns against false security about flu.

The World Health Organization warned on Friday against a false sense of security from waning and apparently mild outbreaks of H1N1 flu, saying the worst may not be over.

WHO Director-General Margaret Chan said there remained "great uncertainty" about the new strain that continues to spread and could pose particular threats in Southeast Asia.

But she could not say whether or when the United Nations agency might raise its pandemic alert to the highest level from the current 5 on a scale of 6. The trigger would be if sustained spread was confirmed in communities outside of North America.

"Actually, I am asking myself that question every day," Chan said in response to a question from Argentina's delegation.

"We are meeting at a time of crisis that could have global implications," she warned the intergovernmental meeting on pandemic influenza preparedness at WHO's Geneva headquarters. "This is a virus so evasive that it can quietly and stealthily move into your country without you even realising it."

The two-day meeting is tackling the sensitive issue of virus sharing in exchange for access to vaccines derived from them.

At the height of fears about bird flu, Indonesia had refused to share H5N1 virus samples without guarantees the vaccines would be provided to poorer countries at an affordable price.

The negotiations, begun in November 2007, have taken on fresh urgency with the emergence of the H1N1 virus.