"These three critical events, these clear threats to international security, have the potential to undo much hard-won progress in public health," WHO Director-General Margaret Chan said on Monday.

Speaking at the opening of the 61st session of the World Health Assembly, which is WHO's supreme decision-making body, Chan said the organisation had identified 21 "hot spots" around the world which are already experiencing high levels of acute and chronic malnutrition. As part of the international task force on the global crisis caused by soaring food prices, WHO is aiming to guide priority action, she said.

Chan said an estimated 3.5 million deaths a year are caused by under-nutrition, and that poor households spend on average between 50 and 75 per cent of their income on food.

"More money spent on food means less money available for health care, especially for the many millions of poor households who rely on out-of-pocket payments when they fall ill."

On climate change, she said that more droughts, floods and tropical storms would add to the demands for humanitarian assistance and would result in a growing number of environmental refugees.

"Again, the poor will be the first and hardest hit. Climate change is already adding an additional set of stresses in areas that are already fragile, with marginal livelihoods and thin margins of survival when shocks occur."

Chan described pandemic influenza, potentially set off by the spread of bird flu to humans, as the "third global crisis looming on the horizon." She said that "the threat has by no means receded, and we would be very unwise to let down our guard, or slacken our preparedness measures."

"As with climate change, all countries will be affected, though in a far more rapid and sweeping way," she added.