"Although some countries have tested their preparedness in exercises ... there is still more testing to be done because when the pandemic does come it will be too late to prepare," he added.

Last year at a similar meeting around $475 million was pledged by donors, but experts played down the apparent shortfall in funding, saying they were confident further funds would be forthcoming.

"A number of countries that might have wished to pledge to this meeting were not in a position ... to organize their pledges on time," Nabarro said.

The United States, the largest donor in the fight against bird flu, raised its pledge to $629 million, an additional $195 million in funding.

For now, humans usually contract the virus only after close contact with infected birds, with the virus killing nearly two-thirds of the people it infects.

But experts worry it may mutate into a form easily transmitted from person to person, leading to a pandemic.

Around a fifth of humanity could fall ill should there be another flu pandemic, according to estimates cited by the World Health Organization, with catastrophic effects on the global economy.

There have been roughly three flu pandemics each century since the 16th century, the WHO says.

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