The number of people with SARS (search) in the latest outbreak in Canada's largest city tripled to 33 on Thursday when health officials broadened their definition of what constitutes a probable case to meet international standards.

The change came after the World Health Organization (search) on Wednesday urged Canada to expand its definition of severe acute respiratory syndrome. Under the old criteria, there were 11 probable cases.

It was not immediately clear how many of the 23 suspected and 50 possible cases under investigation the day before had been reclassified Thursday to bring the number of probable cases to 33.

The new cluster of cases in Canada, first detected last week, was a harsh blow to a health care system that appeared to have brought an initial SARS outbreak in March and April under control. Health officials have told more than 7,000 people to quarantine themselves due to possible exposure.

Elsewhere Thursday, Taiwan reported 50 new cases of severe acute respiratory syndrome, its biggest rise in six days. Officials insisted that Taiwan's outbreak was declining, and said Thursday's jump included 40 patients who earlier tested negative and were later reclassified as confirmed.

China reported just three new cases, the lowest daily number so far.

The global death toll Thursday from SARS was at least 753, out of more than 8,200 people infected since November, mostly in China and Hong Kong.

Because of the new cluster in Canada, Toronto was placed again on WHO's list of SARS-affected areas. Under the adjusted definition, officials Thursday listed 29 cases as suspected and said that 107 other people showing possible SARS symptoms were being monitored.

"I think a large number of them will eventually be suspect or probable cases," said Dr. Donald Low, chief microbiologist at Mount Sinai Hospital and a key figure in Toronto's anti-SARS efforts.

Officials worry the WHO could issue another warning against travel to the city, like one on April 23 that was lifted a week later. WHO spokesman Iain Simpson said Thursday the U.N. agency was not considering another travel advisory yet.

"It's a subjective judgment," Simpson said. "We look at whether there is a high level of risk and if there are things we don't understand before we take any decision."

He added the agency was "watching the situation closely and monitoring it closely."

A total of 29 people have died in the Toronto area since the illness first appeared from Asia in early March in the biggest outbreak outside of Asia. Four probable cases remained hospitalized.

The new SARS cases mean further harm to Toronto's crucial convention and tourism industry. Officials have started aggressive marketing campaigns to lure back visitors after the initial SARS outbreak, including packages of cheap prices for lodging, meal and tickets to theater, ball games and other entertainment.

Organizers announced Thursday that all 70,000 tickets for a June 21 concert to promote Toronto tourism had sold out in three hours. Performers will include Avril Lavigne, the Barenaked Ladies and Sarah McLachlan.

Prime Minister Jean Chretien promised his government would provide financial help to fight SARS in Toronto, and insisted the city was safe to visit.

In response to the new cases, health authorities re-imposed strict controls on Toronto-area hospitals -- closing those where the new cases were found to new patients and limiting access to emergency rooms in all others.

Despite the SARS scare, life has continued as usual in the metropolitan area of more than 3 million people on the north shore of Lake Ontario. No one wears masks in the bustling downtown streets, and restaurants, theaters and other entertainment venues remain open despite complaints of decreased business because of SARS.

Health officials said the increased figure for new probable SARS cases was expected because of the altered definition.

"They're getting higher but they're not higher per day at this period of time," said Dr. James Young, the Ontario commissioner of public safety.

"We're not seeing large number of cases coming into the system each day."

On Wednesday, the WHO advised Canada to broaden its definition of probable SARS following concern that the current one provided an incomplete accounting of the situation.

The Health Canada Web site defined a probable case as showing a severe progressive respiratory ailment. WHO has a less restrictive definition on its Web site, only requiring that a respiratory illness be visible on X-rays.

The quarantines include 1,700 students and staff at Father Michael McGivney Catholic Academy (search) in Markham, a northern suburb of Toronto, where a student showed symptoms of SARS while going to classes for three days last week. Health officials closed the school until June 3.

Guidelines for home quarantine include no visitors, sleeping in a separate room from anyone else and wearing a respirator mask when in contact with others.