The confirmed tally of swine flu cases worldwide has risen to 10,243, including 80 deaths, the World Health Organization said Wednesday.

The United States accounted for most of the increase, where there are now 5,469 cases, the WHO said in its daily update on the outbreak of H1N1 influenza in 41 countries.

"There is an increase of 413 cases in the past 24 hours, with most in the United States with 346 new cases," WHO spokesperson Fadela Chaib told reporters.

Japan's Health Ministry confirmed its first case in Tokyo, a 16-year-old female high school student who had recently returned from New York. Another 51 cases have been reported in Japan, the WHO said.

As of Wednesday, there were 719 confirmed cases of swine flu in Canada, up from 496 reported on May 15, according to the Public Health Agency of Canada. Most of the new lab-confirmed cases were in Ontario and Quebec.

The UN health agency is closely watching for any signs the virus is spreading in a sustained manner in a second region outside of North America.

If so, WHO could raise its alert level from the current Phase 5, which indicates a global outbreak is likely, to Phase 6 — an epidemic occurring in at least two worldwide regions.

The swine flu outbreak has taken over the agenda at the WHO's annual meeting in Geneva this week. Health ministers have urged the agency to consider the severity of the virus in their deliberations on the alert level.

Risk factors probed for hospitalized patients

Of the 80 deaths worldwide, 72 have been in Mexico. Elsewhere, patients have shown relatively mild symptoms.

On Tuesday, the U.S. Centers for Disease Control released a study of 30 hospitalized swine flu patients that suggested obesity may increase the risk of having a severe case.

"We were surprised by the frequency of obesity among the severe cases that we've been tracking," said CDC spokesperson Dr. Anne Schuchat.

"The question of whether people with obesity need to be treated differently in terms of antiviral treatment or our seasonal flu vaccine recommendations is one we're looking into right now."

"If there truly is an increased risk of severe complications in obese patients, it would be important to take steps to attend to that."

It's not yet clear whether obesity makes a patient more vulnerable, of if the numbers reflect how common obesity is in the U.S. population, according to CDC.

Most severe cases in the U.S. were among people who had a medical condition such as chronic lung conditions, suppressed immune systems, heart disease or diabetes that increased their risk compared with healthy people.

About 17 per cent of hospitalized patients were pregnant. The immune system is known to be suppressed during pregnancy, and pregnancy can increase the likelihood of complications from any kind of influenza.

Decreased lung capacity during pregnancy may also play a role, Schuchat speculated.

With files from The Canadian Press