By Stephanie Nebehay and Amran Abocar

BEIJING/TORONTO (Reuters) - Saying SARS was still spreading out of control, the World Health Organization tightened travel advisories on Wednesday, adding Beijing and Toronto to the list, while authorities in Beijing started quarantines.

WHO said someone infected with Severe Acute Respiratory Syndrome had traveled from Toronto to another country -- it would not say where -- and infected five people there.

And WHO cited the spread of SARS in Beijing and Shanxi provinces as the reason for adding those areas to an existing advisory about travel to Hong Kong and the southern Guangdong province.

The Beijing city government said it would quarantine people and buildings infected, or suspected of being infected, with the SARS virus, the official Xinhua news agency said -- by force if necessary.

The government also ordered all Beijing's primary and secondary schools closed for two weeks from Thursday, a move that will affect an estimated 1.7 million children.

Beijing, a city of 14 million people, has reported almost 700 SARS cases and 35 deaths. Shanxi, which lies west of the capital, has the third highest number of cases in China, 157 cases with seven deaths, according to health ministry figures.

SARS, a sometimes deadly respiratory infection caused by a relative of a common cold virus, has killed at least 251 people worldwide and infected 4,200 more since it appeared in Guangdong late last year.

POSTPONING TRAVEL TO TORONTO

It has spread to virtually every continent but outside of Asia, Canada has been especially hard-hit. Fifteen people have died from SARS in Toronto and 324 probable or suspect cases have been reported nationwide.

On Tuesday the U.S. Centers for Disease Control and Prevention issued a travel advisory for Toronto, saying people should not postpone travel there but should be aware of SARS.

But WHO expressed stronger concern.

"We are going to recommend that people who have unnecessary travel to Shanxi, to Beijing, and to Toronto postpone that travel if possible," WHO's Dr. David Heymann told reporters in a telephone briefing.

"Toronto has had last week an exportation which set up a cluster of 5 cases ... and I cannot tell you the country ... but this is what has called it again to our attention," he added.

"The outbreak in this area has continued to grow in magnitude and has affected groups outside the initial risk groups of hospital workers, their families and other close person-to-person contacts."

Canadian health officials and local politicians were outraged.

"Where did this group come from? Who did they see? Who did they talk to?" a visibly angry Mel Lastman, Toronto's outspoken mayor, asked at a news conference.

"Let me be clear. If it's safe to live in Toronto, it's safe to come to Toronto. I dare them to be here tomorrow."

City health officials said the WHO advisory was both irresponsible and regrettable. "The outbreak is not over but it is definitely under control," said Sheela Basrur, Toronto's medical officer of health.

FEARS SARS COULD BECOME PERMANENT

WHO fears that SARS, with a mortality rate that approaches 6 percent, may become a permanent human disease. It has a higher mortality rate than influenza, which kills an average of 250,000 people every year globally, although SARS so far affects far fewer people.

But health experts say a contagious respiratory disease could easily become as common as influenza is and, with such a high mortality rate, it would be devastating.

The best hope of containing SARS, they say, is to act quickly and decisively.

"Today, one of the most important means of spreading diseases around the globe is air travel," Heymann said.

"That is why we have been concentrating on that as a part of it. That is where I would put us today. We still do not know whether or not we can stop this disease from becoming endemic."

Hong Kong announced a $1.5 billion package to help businesses reeling from the impact of the disease. The city has now had 105 SARS deaths.

In Singapore, where there have been 189 infections and up to 17 deaths, alarm was growing over an outbreak among vendors at the city-state's largest vegetable market and the government threatened to jail people violating quarantine.

Taiwan reported nine new cases as a raft of infections at a Taipei hospital brought the island's total to 38 and raised concern the virus could spread into the community.

The World Trade Organization said the epidemic would contribute to a gloomy year with global trade volume expected to increase by less than three percent after an already poor 2.5 percent rise last year.