In its first warning that suggests a deadly flu-like illness can be spread on airplanes, the World Health Organization said Thursday that passengers with symptoms of the disease or who may have been exposed to it shouldn’t be allowed to fly.

Airlines flying out of cities where the mystery disease is spreading should question passengers at check-in desks for signs of severe acute respiratory syndrome, or SARS, the global health agency said.

“If the passengers are sick, health workers will be recommending to the airline that they not board the plane,” said Dr. David Heymann, WHO’s infectious diseases chief.

The advice from the WHO — which can only make recommendations to governments — is directed at flights leaving Toronto; Singapore; Hanoi, Vietnam; Hong Kong; Taiwan; Beijing, Shanghai and the Chinese province of Guangdong, where the earliest cases of SARS occurred.

While suspected cases have been reported in more than a dozen countries, the illness is not considered to be spreading in most communities. So far, the WHO says, 1,408 people have fallen ill with SARS and 53 people have died; that doesn’t include a death Thursday that Hong Kong officials were reporting.

The United States has 51 suspected cases, said Dr. Julie Gerberding, head of the U.S. Centers for Disease Control and Prevention.

The death rate has remained around 4 percent since the outbreak began, experts said. There have been three deaths in Canada and none in the United States.

Thursday’s advice from the WHO was the latest broad attempt at slowing the spread.

“The recommendation still remains that there should be no interruption in travel or trade, but we’re shifting a little bit more responsibility to countries where there are these infected areas,” Heymann said.

However, the CDC’s Gerberding encouraged Americans to defer vacations to Asia if they were able.

“This is now a global epidemic and potentially a global pandemic” if it’s not quickly brought under control, she said.

The WHO has teams of infectious disease experts in the affected countries. But some places continue to have problems containing the disease, which apparently got its start last winter in Guangdong province in China. It is believed to be a virus and there is no treatment for it, although medicines are being tested.

In Hong Kong, where at least 10 people have died, the government said Thursday it would quarantine more than 1,000 people and close its schools.

In Ontario, Canada’s most populous province, health authorities declared a state of emergency and called for a 10-day quarantine of people who had visited a hospital where the outbreak spread — a number that could be in the thousands.

Ontario health officials said Thursday they had bought all the high-grade surgical masks available in Canada to protect medical workers.

“One hundred thousand were delivered yesterday, 200,000 will be delivered today and the last 40,000 were purchased early this morning,” said Dr. Colin DCunha, Ontarios Commissioner of Public Health.

Canadian Health Minister Anne McLellan said she was taking the WHO screening recommendation “very seriously” and her office has already started working with the airlines.

Health officials already knew the disease had spread beyond Asia by international air travel on March 15, when the Canadian cases turned up. But at the time it was not clear whether those people were sick on the plane or got sick after coming home.

“Now we know that there are people who are traveling when they are sick,” Heymann said. “There have been more and more cases who have traveled on airplanes and we aren’t yet 100 percent sure of how this disease is spreading.”