WHO's flu chief Keiji Fukuda said the agency wanted to avoid "adverse effects" if it announces a global outbreak of swine flu. Fukuda said people might panic or that governments might take inappropriate actions if WHO declares a pandemic.

Some flu experts think the world already is in a pandemic and that WHO has caved in to country requests that a declaration be postponed.

"On the surface of it, I think we are in phase 6," or a pandemic, said Margaret Chan, WHO's director-general.

Chan said it was important to verify the reports that the virus is becoming established outside North America before 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," she said.

Chan said she will hold a conference call with governments Wednesday in order to verify some of the reports she has received before making a formal announcement. "Once I get indisputable evidence, I will make the announcement," she told reporters in Geneva.

WHO said the virus has infected 26,563 people in 73 countries and caused 140 deaths. Most of the cases have been in North America, but Australia also has seen a sharp increase in recent days.

In most of the 73 countries, the new H1N1 virus has triggered only mild illness. But the fact that some of the deaths have occurred in otherwise healthy adults has prompted WHO to classify the outbreak as "moderate" for the time being.

"Approximately half the people who have died from this H1N1 infection have been previously healthy people," Fukuda said, adding that this was "one of the observations which has given us the most concern."

Wealthy countries such as the U.S., Canada and Britain already have large stockpiles of antivirals used to treat swine flu, but many developing countries have no supplies of the drugs and could be more vulnerable to the virus, given their struggle with widespread problems such as AIDS, malnutrition and malaria.

Some pharmaceutical companies are preparing to make a swine flu vaccine, if WHO declares a pandemic.

The number of cases in Australia jumped to more than 1,000 by Monday, with the vast majority reported from the southern state of Victoria.

If the swine flu virus were to be shown to be spreading rapidly from person to person in another world region beyond the Americas, such as Australia or Europe, that should trigger the conditions for WHO to declare a pandemic, meaning the outbreak has gone global.

"We are getting really very close to knowing that we are in a pandemic situation," Fukuda said.

He also said it was more important that countries take "the right actions" than that they accurately report the extent of their outbreaks.

With 675 reported swine flu cases in Britain, some experts suspect the virus already is entrenched in communities, but that U.K. health authorities are deliberately not testing for the virus and not reporting cases.

In recent weeks, two Greek students who caught swine flu in Scotland had no history of contact with any confirmed cases, a clear sign the virus is spreading in British communities.

"Our primary concern is not so much the numbers that are being reported," Fukuda said. He said countries simply needed to take appropriate actions to handle their outbreaks.

In his weekly update on the outbreak, Fukuda also addressed reports that an unusually large number of severe cases have occurred among Canada's Inuit population.

"There are reports of infections occurring in Inuit communities with a disproportionate number of serious cases," he said. "These are observations of concern to us."