GENEVA - THE World Health Organisation said on Wednesday it is working to mathematically model the spread of swine flu, as concern grows that actual case numbers may be far higher than the agency's tally of officially diagnosed infections.

WHO brought together over 20 independent experts for the three-day meeting that began on Wednesday to better understand how the outbreak developed from a handful of cases to a global epidemic in less than two months, said spokeswoman Aphaluck Bhatiasevi.

'They will be working together to describe and predict the behavior and impact of the pandemic, and demonstrate potential outcomes of proposed intervention efforts,' Ms Bhatiasevi told reporters.

According to WHO's latest update on Wednesday, a total of 77,201 confirmed cases and 332 deaths have been reported in over 110 countries.

But US health officials said last week the number of Americans infected with the new A(H1N1) virus may be as high as 1 million. The estimate by the US Centre for Disease Control and Prevention was based on mathematical modeling and surveys by health officials.

'We're now probably off by orders of magnitude in terms of the real number of cases versus the number of diagnosed cases,' said Andrew Pekosz, a flu expert at Johns Hopkins University in Baltimore. -- AP