Canadian officials angrily said they would challenge the health advisory and declared their nation's largest city still "a safe place." Toronto is the first location outside of Asia targeted in efforts to contain the disease.
In Beijing, Chinese officials said all public schools would close Thursday for two weeks, affecting 1.7 million children. Thousands of people trying to flee the outbreak packed the capital city's train station and airport.
A major medical center in China's capital, the People's Hospital of Peking University, was closed Thursday amid a SARS outbreak. More than 2,000 employees were under observation for the disease while the hospital was being disinfected.
Dr. David Heymann of the World Health Organization (more news | Web) said the new travel alert, which includes China's Shanxi province, was necessary because "these areas now have quite a high magnitude of disease, a great risk of transmission locally ... and also they've been exporting cases to other countries."
The advisory, which says any unnecessary travel to those locations should be postponed, will be reviewed again in three weeks, he said. Previously, WHO warned against non-urgent travel to Hong Kong and the Chinese province of Guangdong, where the virus was first reported last November.
Dr. Paul Gully, director general of Health Canada, said he would challenge WHO's assertion in a letter.
"Toronto continues to be a safe place," he said.
Dr. Clifford McDonald, an official with the U.S. Centers for Disease Control and Prevention, noted that the CDC had not issued the same strong advice. The CDC has warned travelers to take precautions when visiting Toronto.
And Toronto medical officer Dr. Sheela Basrur said the outbreak, while serious, "is contained -- largely in hospitals which is, frankly, where it belongs. So we don't have widespread community spread."
But Heymann, WHO's communicable disease chief, said Toronto had not contained the disease. A major reason for WHO's action, he said, is that a cluster of SARS cases among health workers in another country was traced to the Canadian city in the last week.
He would not say where the new cluster emerged, but there have been reports of at least three incidents of SARS being exported from Toronto. One involved a Toronto medical assistant who apparently spread SARS to her family in Manila before she died of it.
That case is the only reported one where an infected person from Canada is known to have triggered SARS in another jurisdiction.
Dr. Colin D'Cunha, Toronto's commissioner of public health, said that means Toronto is "an exporter of sorts," but not enough for a travel advisory.
There is no treatment for severe acute respiratory syndrome, which has symptoms similar to pneumonia. It has killed at least 250 people worldwide, out of more than 4,000 infected.
Canada has been the most affected area outside Asia, with 140 cases and 16 deaths as of Wednesday, all in the Toronto area.
The disease has meant disaster for many businesses. Asia's aviation industry is in its worst crisis ever, some analysts say, with layoffs and thousands of flights canceled.
Hong Kong Chief Executive Tung Chee-hwa announced a $1.5 billion relief package Wednesday to try to help the territory's battered economy bounce back.
Even Chinatown restaurants in American cities have lost customers, despite the fact that there are fewer than 40 U.S. cases and no one has died.
In Toronto, near the New York state border, Wednesday's warning from WHO immediately piled on the misery. Mayor Mel Lastman was furious at the health agency.
"I've never been so angry in all my life," he said, adding that he was "shocked that the medical evidence we have before us does not support this."
As a result of the warning, Major League Baseball urged players to take precautions -- such as avoiding crowds and contact with fans -- when visiting the home of the Blue Jays through the All-Star break in mid-July.
"The ripple effect is huge because the hotel industry, the restaurant industry, sporting events, everything filters out of that," said Rick Naylor, head of Accucom, a company that organizes trade shows to Toronto. "It's not just the conventions."
Deputy Mayor Case Ootes said people should not fear coming to Toronto.
"I can assure you that the situation here is totally different than it is in the Far East," he said Wednesday in an interview PBS' The NewsHour. "People in Toronto are going about their business as usual."
Ted Carmichael, chief Canadian economist for J.P. Morgan in Toronto, said, "What is uncertain is the duration of the economic impact of SARS. If the outbreak is not contained soon, the negative effect on consumer confidence and business spending are likely to increase."
Economic activity in the greater Toronto area accounts for about 20 percent of the national gross domestic product, according to David Dodge, governor of the bank of Canada.
In Beijing, along with the school closing, authorities said they would quarantine people exposed to the SARS virus and restrict access to buildings where there are infections. They did not say how they would enforce the measures.
The number of SARS cases reported by Beijing has leaped from fewer than 40 to nearly 600 in just four days. However, world health officials say the spike could be caused by changes in the way SARS cases are reported.
At the Beijing Train Station on Wednesday, thousands of workers and students in white gauze masks waited to board trains to go home. Capital Airport also was more crowded than normal.
A high school student who only gave her surname, Chen, said she came to Beijing to take a college entrance exam two weeks ago and was anxious to return to her southern home province of Yunnan, which shares a border with Myanmar.
"I really can't wait to get out of here because of SARS," Chen said. "Beijing is too dangerous. It's safer back home."