With growing reports of swine flu cases across the United States, the acting director of the Centers for Disease Control and Prevention said he expects deaths from the outbreak. But so far, all of the cases have been mild.

"I fully expect we will see deaths from this infection," Richard Besser said in Atlanta on Tuesday.

There are at least 66 confirmed cases in five states, including about 45 in New York. But New York City Health Commissioner Thomas Frieden warned that "many hundreds" of schoolchildren may be infected with the virus.

Meanwhile, New York City Mayor Michael Bloomberg announced that two people, a child in the Bronx and an adult in Brooklyn, were in hospital. But those cases were separate from the larger suspected outbreak.

He also said that authorities are investigating potential cases at a special education school in Queens.

At the same time, authorities in California said they are investigating one death that may be linked to the virus. Initially, they were investigating two deaths, but one has since been ruled out.

The suspected infections added to an ever-growing number of cases around the world Tuesday as officials waited to hear if the virus is beginning to spread in a sustained way from person-to-person.

The World Health Organization (WHO) said Tuesday it was waiting to hear formal confirmation from U.S. health officials about how the disease was spreading in that country.

"It appears, and I think we're still awaiting for a final confirmation from the U.S. authorities, but it appears that there's a number of cases in New York which appear to be human-to-human transmission," WHO spokesperson Gregory Hartl told a news briefing.

Hartl said such secondary transmission of the virus was "probable."

He also said if the U.S. or Canada does make such a confirmation then the WHO could raise the alert level from Phase 4 to Phase 5 (out of 6), which would indicate a high probability of a pandemic.

Mexico

The virus was first reported in Mexico, which has suffered the most from the outbreak. At least 159 people are suspected to have died because of swine flu.

In Mexico City, crowds have stayed away from movie theatres, museums and public transit. Authorities have also shut down schools, also told restaurants to only serve take-out food.

Arturo Mendicuti, president of the city's Chamber of Trade, Services and Tourism, estimates the outbreak is costing Mexico City at least US$57 million every day. But that figure was released before the government expanded its containment measures.

"Certainly, there is an epidemic here of fear and frustration in addition to the very real fear you see everywhere here of the swine flu itself," CTV's Tom Walters reported Tuesday from Mexico City.

Outside Mexico, no deaths from swine flu have been confirmed.

Daniel Epstein, a spokesman for the Pan American Health Organization, said Tuesday that it's unclear why only Mexicans have died so far.

"We are looking into it, it's a very perplexing situation," Epstein told CTV Newsnet.

By Tuesday afternoon, the number of swine flu cases in Canada had reached 13. On the other side of the planet, New Zealand confirmed that 11 people who recently came back from Mexico had contracted the virus.

New Zealand Public Health Director Mark Jacobs said those infected had only suffered "mild illness" and were expected to recover.

Officials are dealing with 43 more suspected cases in the country.

Meanwhile, Israel's Health Ministry confirmed one case of swine flu Tuesday in the city of Netanya. The victim, 26, recently returned from a trip to Mexico, Health Ministry spokeswoman Einav Shimron said.

Officials said the man has already recovered but is being kept in hospital until the Health Ministry decides he can leave.

As swine flu spreads, authorities are focusing less on trying to contain the virus.

"Containment is very difficult now if not impossible so there are measures we can take to mitigate," World Health Organization spokesman Paul Garwood told CTV's Canada AM on Tuesday.

Swine flu, also known as A H1N1, is normally contracted through contact with pigs.

However, this strain is spread through human-to-human contact.

Symptoms include fever, muscle and joint pain, cough, sore throat, eye pain, shortness of breath and extreme fatigue.

With files from The Associated Press