NEW DELHI (AFP) - The global cost of a possible bird flu pandemic could be up to two trillion dollars, a top World Bank official said Tuesday.

The risk of a pandemic was still as great as it was two years ago despite improvements in the capacity of many countries to respond to the infection, a joint report by the United Nations and World Bank said last week.

"The global economic costs could be between 1.5 to two trillion dollars," Peter Harrold, acting Vice President of the World Bank, told an international conference on avian flu in New Delhi that wraps up Thursday.

International donors have pledged 2.3 billion dollars to help countries combat the threat, and more than one billion dollars had been allocated to other groups involved in the fight, Harrold said.

More than 600 delegates from 105 countries are in New Delhi to discuss preparedness and challenges in fighting avian flu.

Experts fear a virus mutation that could result in severe and easily transmitted influenza in humans could create the next pandemic, with far-reaching consequences.

"About 20 percent of the global population will be affected during the next pandemic," Margaret Chan, director general of the World Health Organisation, told the gathering.

Chan said 28 million people may need medical care over a relatively short period and worker absenteeism could reach 35 percent of the work force.

But experts said considerable progress had been made in preparing for a pandemic.

"Ninety-five percent of the countries report that they have developed pandemic preparedness plans," said David Nabarro, UN Systems Co-ordinator for Avian Influenza and one of the authors of the UN-World Bank report.

But the response was still "patchy," and "entrenched" infection in some countries continued to pose a major threat to human health, Nabarro said.

Once the virus is entrenched, eliminating it becomes tougher and there is greater risk of humans contracting the infection, experts say.

The H5N1 strain of bird flu has killed more than 200 people worldwide since late 2003. But the number of human infections and deaths declined this year over last year.

Forty-eight people died of the infection in 2007, down from 71 in 2006, according to the WHO, and experts said outbreaks were also being detected more rapidly and responses have become more effective.

Twenty-six countries reported flu outbreaks in birds in 2007, of which four -- Bangladesh, Ghana, Saudi Arabia and Togo -- experienced them for the first time.