The World Health Organization said Saturday the virus responsible for the swine flu outbreak among humans doesn't appear to be spreading outside of North America, but cautioned that a pandemic still can't be ruled out.

Michael Ryan, director of Global Alert and Response of the World Health Organization, looks on before a news conference about the swine flu Saturday in Geneva. (Keystone/Martial Trezzini/Associated Press)"At the present time I would still propose that a pandemic is imminent, because we are seeing the disease spread," Michael Ryan, the World Health Organization's director of Global Alert and Response, told reporters in Geneva.

But a sustained spread of the outbreak among communities outside North America would be required before raising the pandemic alert level to the highest category, he said. So far there is "no evidence" that has happened, Ryan said.

The alert level stands at five on a six-point scale. A Level 5 alert means there has been human-to-human spread of a virus in at least two countries in one region, according to the organization's pandemic response guidelines.

The classification means a pandemic is imminent and countries must finalize preparations to deal with the outbreak, officials said. Phase 6 means a pandemic has occurred.

"At this point we have to expect that Phase 6 will be reached. We have to hope that it is not reached," he said.

Suspected deaths in Mexico down

Meanwhile, Mexico revised the number of suspected deaths from the swine flu virus down to 101 from 176 early Saturday.

Mexico's health secretary also said the number of confirmed swine flu cases in that country has risen to 473, including 19 people who died.

Ireland reported its first confirmed case Saturday, as did Costa Rica and Italy.

Fifteen other countries have reported 615 cases, Ryan said Saturday.

The virus is confirmed to have killed at least 17 people: 16 in Mexico and one in the United States, according to the WHO. In Mexico, "about half of the cases are [people who are] under 21 and half the cases are over 21 and [are] spread across different age groups," said Ryan.

The U.S. Centers for Disease Control and Prevention confirmed Saturday that there were 160 cases reported in 21 states. In the United States, the median age of the confirmed cases is about 17, said the CBC'S Catherine Mercier, reporting from New York.

The virus, which has been identified as a new strain of the H1N1 subtype of type A influenza, is believed to have originated in Mexico and has since appeared in the U.S., Canada, Europe and elsewhere.

In Canada, the total number of confirmed cases rose to 85 as health officials in Nova Scotia, British Columbia, Alberta, Ontario and Quebec confirmed a total of 30 new cases Saturday.

All the Canadian cases are considered mild.

Ryan said the newly reported cases in Mexico "reflect primarily ongoing testing of previously collected samples and not a surge of people falling sick."

Mexico takes precautions

Mexico has taken extraordinary measures to combat the epidemic, ordering all non-essential government and private businesses to shut down for five days.

In the wide valley where 20 million people live in and around the capital, streets were strangely quiet Saturday, its usually crowded markets shuttered and even parks locked down.

"What we thought is that we would have an exponential growth in the number of persons with symptoms. But the information we have is that hasn't occurred, and we now have a stabilized curve with no important growth" of confirmed cases, Mexico City Mayor Marcelo Ebrard said Saturday.

"That is the big question: Is it stabilizing or not? And it is too early to say, but I think we are getting systems in place where we are going to be able to get a handle on this soon," said Dr. Steve Waterman of the U.S. Centers for Disease Control and Prevention.

Ryan also announced Saturday that the WHO would send 2.4 million doses of antiviral drugs to countries around the world, including Mexico. The least developed countries with the poorest health systems would be given priority, he said.

With files from The Associated Press