Many countries could see swine flu cases double every three to four days for several months until peak transmission is reached, once cold weather returns to the northern hemisphere, said WHO's Western Pacific director, Shin Young-soo.

"At a certain point, there will seem to be an explosion in case numbers," Shin told a symposium of health officials and experts in Beijing. "It is certain there will be more cases and more deaths."

The WHO says the swine flu virus — also known as H1N1 — has killed almost 1,800 people worldwide, and has declared a pandemic.

International attention has focused on how the pandemic is progressing in southern hemisphere countries such as Australia where winter — and the flu season — has started.

But it is in developing countries that the accelerated spread of swine flu poses the greatest threat as it places underequipped and underfunded health systems under severe strain, Shin said.

WHO earlier estimated that as many as 2 billion people could become infected over the next two years — nearly one-third of the world's population.

Others said Shin's cautionary comments were needed but that they were optimistic the spread would not be that serious.

Ann Moen, an influenza expert with the U.S. Centers for Disease Control and Prevention, said that if current trends continue it is possible that the swine flu pandemic will not be worse than a severe flu season.