The head of the World Health Organization warned Tuesday the worldwide H1N1 flu outbreak appears to have reached pandemic proportions.

In Geneva, WHO Director-General Margaret Chan said "on the surface of it, I think we are in phase 6," or a pandemic. The WHO expressed particular concern about Canada.

While the WHO has yet to officially declare a global pandemic, many flu experts say this is already the case.

Chan said it was important to verify the reports that the virus is becoming established outside North America before raising the alert level and declaring a pandemic.

"The decision to make a phase 6 announcement is a heavy responsibility, a responsibility that I will take very seriously, and I need to be convinced that I have indisputable evidence," Chan said.

On Wednesday, the WHO will hold a special conference with international health officials after which a formal announcement may be made.

WHO officials say they're wary of panicking populations around the world.

"One of the critical issues is that we do not want people to panic if they hear that we are in a pandemic situation,'' Feiji Fukuda, WHO's acting assistant director-general, said in a media call Tuesday.

"We know the virus is spreading and we are now seeing activity picking up in a number of countries. We know that we are getting closer to probably a pandemic situation,'' Fukuda said.

Canada has the third-highest number of cases worldwide (2,225), behind the United States (13,217) and Mexico (5717).

There have been four H1N1 flu deaths in Canada, and 121 hospitalizations.

The WHO head expressed concern about the "disproportionate number'' of serious cases on a remote native reserve in northern Manitoba.

Hundreds of aboriginals in the community of St. Theresa Point (population 3,200) -- 500 kilometers northeast of Winnipeg -- have reported symptoms, more than a dozen have been treated in hospital, several are reported to be on ventilators.

"These are observations of concern to us,'' Fukuda said.

Speaking on CTV News Channel, infectious disease specialist Dr. Neil Rau called the Manitoba outbreak "puzzling."

"What I find unusual here with the Manitoba situation is that they're describing so many at once. Genetic susceptibility of people of First Nations background is being raised as an explanation," Rao explained in Toronto.

"But no other flu virus has disproportionately affected one ethnic group over another historically so this would be a really new turn if that's the case here."

Experts say -- globally -- the disease pattern is different from what officials see with a normal flu outbreak.

Victims include not the very young, the very old and the very sick. Perfectly healthy people in their prime are falling ill.

The new H1N1 flu, a mixture of swine, bird and human viruses, has now infected 25,288 people in 73 countries, with 139 deaths.

With files from The Associated Press