Toronto ? Hospital workers in Toronto once again strapped on stuffy masks and gowns Saturday to confront a new possible SARS outbreak that officials said involved 33 suspected cases, weeks after Canada proclaimed itself free of the deadly virus.

The new cluster of possible cases of severe acute respiratory syndrome prompted U.S. health officials to issue a new travel alert for Canada’s largest city.

The World Health Organization confirmed one new positive case but said more confirmation of an outbreak was needed before considering such a travel warning.

In Toronto, health officials ordered restricted access and use of protective masks and gowns for all area hospital emergency rooms, repeating steps taken earlier this year against the largest SARS outbreak outside of Asia.

At least 500 people possibly exposed were told to quarantine themselves at home for 10 days as a precaution, they said.

Dr. Colin D’Cunha, the Ontario commissioner of public health, said 33 people with respiratory illness were being tested for SARS. All were believed to have contracted the illness in hospitals in the past month, he said.

Two of the suspected cases involved elderly patients who died in recent weeks. If confirmed, they would raise the SARS death toll in the Toronto area to 26.

A pilot is shown on the monitor of a thermal scanner at Pearson International Airport in Toronto, Canada. Health Canada said Friday that it now had thermal scanners, used to detect people with a high fever, up and running at the airport.

A formal definition of a probable SARS case requires a link to a known SARS case, and that has yet to be established, D’Cunha said. But the officials were proceeding as if all the possible new cases were SARS.

“Clinically, we think they have it,” said Dr. Donald Low, a microbiologist at the forefront of Toronto’s anti-SARS efforts.

Despite the inability to trace the new suspected cases to known cases, officials said there was no danger of an uncontrolled outbreak.

“This is not a disease out there in the general community,” D’Cunha said.

A WHO spokesman in Geneva said one new case in Toronto tested positive, and results on other cases were expected shortly. Dick Thompson said Canadian officials reported 31 cases of respiratory illness they were checking for possible SARS. The figure he was given excluded the two dead patients.