Margaret Chan, centre, director general of the World Health Organization, speaks next to U.S. Health and Human Services Secretary Kathleen Sebelius, left, and Mexican Health Minister Jose Angel Cordova Villalobos, right, on May 18. (Keystone, Laurent Gillieron/Associated Press)

The World Health Organization will hold an emergency meeting of experts on Thursday to discuss a spike in swine flu cases worldwide as the agency weighs declaring a pandemic.

WHO Director General Margaret Chan will hold a conference call with experts on the state of the outbreak, spokesman Thomas Abraham said.

Abraham said Chan spoke to "seven or eight" unnamed countries about their cases of H1N1 swine flu.

Chan told reporters on Tuesday that she believes a pandemic is occurring and she is seeking "indisputable evidence" that the H1N1 virus is spreading from person to person outside of North America, where the outbreak began, before declaring a pandemic.

Moving to Phase 6 or pandemic status does not mean the situation is more severe or that people are getting seriously sick in higher numbers, said Keiji Fukuda, acting WHO assistant director general, noting the agency still considers the impact on countries to be "relatively moderate."

WHO defines a pandemic in terms of its geographic spread.

More than 26,500 people have been infected worldwide and about 140 have died of swine flu.

By late Wednesday, Australia had reported 1,260 cases, up from 876 reported on June 5. Australian health officials said many cases cannot be traced back to travellers or common infection sites such as schools, which suggests the virus is now circulating in the community as it is in Canada, the U.S. and Mexico.

'Doesn't change anything'

As of Monday, 2,446 laboratory-confirmed cases of H1N1 flu virus have been reported in all provinces and territories in Canada except Newfoundland and Labrador, according to the Public Health Agency of Canada. There have been four deaths related to swine flu in Canada.

"Going to Phase 6 tomorrow, for example, doesn't change anything of what we're doing," Dr. Michael Gardam, director of infectious diseases prevention and control at the Ontario Agency of Health Protection and Promotion, said in an interview Wednesday.

"It all comes down to how transmissible and severe it is."

Going to Phase 6 could help by forcing people to pay attention to swine flu and think about what their contingency plans would be if many fall sick, such as working from home and considering child-care options if schools close, Gardam said.

Looking for community spread

Swine flu clearly isn't in the same category as the 1918 pandemic in terms of deaths and illness, but the jury is still out on how many people may fall sick and what the effects on the health-care system could be, Gardam added.

Flu experts have questioned whether the virus is circulating more widely than the official tallies suggest.

Britain, for example, has reported 675 cases, and the British Health Protection Agency denies that swine flu is established in communities, meaning random cases are occurring that can't be traced back to the first infected person.

In the May 28 issue of Eurosurveillance, researchers reported two confirmed cases of swine flu among Greek men returning from Scotland, and raised questions about whether more sustained human-to-human transmission is occurring in Scotland.

WHO spokesman Dick Thompson said the agency is trying to resolve differences between media reports from several countries suggesting more cases or wider transmission than the official reports.

With files from The Associated Press