BEIJING--China closed off a city of more than 11 million people Thursday in an unprecedented effort to try to stop the spread of a deadly new virus that has sickened hundreds and spread to other cities and countries in the Lunar New Year travel rush.

Police, SWAT teams and paramilitary troops guarded Wuhan’s train station as last-minute travelers arrived, only those holding tickets for the last trains allowed to enter. At exactly 10 a.m., metal barriers blocked entrances while helpless would-be travelers were turned away, with some complaining they had nowhere to go.

Virtually everyone at the scene was wearing masks, news website The Paper’s live broadcast showed. People have been lining up to buy them at pharmacies, which limited sales to one package per customer. Medical workers wore protective suits outside a hospital where some patients with the viral respiratory illness are being treated.

“To my knowledge, trying to contain a city of 11 million people is new to science,” Gauden Galea, the World Health Organization’s representative in China, told The Associated Press in an interview at the WHO’s Beijing office. “It has not been tried before as a public health measure. We cannot at this stage say it will or it will not work.”

Local authorities have demanded all residents wear masks in public places and urged government staff to wear them at work and for shopkeepers to post signs for their visitors, Xinhua quoted a government notice as saying.

“Those who disregard the warning will be punished according to relevant laws and regulations,” the notice said.

Virtually no one would be allowed to leave Wuhan, an industrial and transportation hub in central China’s Hubei province. Train stations, the airport, subways, ferries and long-distance shuttle buses were closed, according to the state Xinhua News Agency. It cited the city’s anti-virus task force as saying the measures were taken in an attempt to “effectively cut off the virus spread, resolutely curb the outbreak and guarantee the people’s health and safety.”

Li Bin, deputy director of the National Health Commission, had warned people in Wuhan on Wednesday to avoid crowds and public gatherings.

The illnesses from a newly identified coronavirus first appeared last month in Wuhan, and the vast majority of mainland China’s 571 cases have been in the city. Other cases have been reported in the Thailand, the United States, Japan and South Korea. One case was confirmed in the southern Chinese territory of Hong Kong after one was earlier confirmed in Macao. Most were people from Wuhan or had recently traveled there.

A total of 17 people have died, all of them in and around Wuhan. Among the victims, the average age was 73, with the oldest aged 89 and the youngest 48.

The significant increase in illnesses reported just this week come as millions of Chinese travel for the Lunar New Year, one of the world’s largest annual migrations of people. Analysts have predicted that the reported cases will continue to multiply.

“Even if (the number of cases) are in the thousands, this would not surprise us,” Galea said, adding, however, that the number of cases is not an indicator of the outbreak’s severity, so long as the mortality rate remains low.

The coronavirus family includes the common cold as well as viruses that cause more serious illnesses, such as the SARS outbreak that spread from China to more than a dozen countries in 2002-2003 and killed about 800 people, and Middle Eastern respiratory syndrome, which developed from camels.

The first cases in the Wuhan outbreak were connected to people who worked at or visited a seafood market, which has since been closed for an investigation. The head of the Chinese Center for Disease Control said the outbreak may have resulted from human exposure to wild animals at first but the virus also may be mutating. Mutations can make it deadlier or more contagious among people.

“We are still in the process of learning more about this disease,” Gao Fu, the CCDC head and an academician of the Chinese Academy of Sciences, said at a Wednesday news conference.

Also Wednesday, the WHO put off deciding whether to declare the outbreak a global health emergency because it needs more information. Another meeting is set for Thursday.

WHO defines a global emergency as an “extraordinary event” that constitutes a risk to other countries and requires a coordinated international response.

Some countries have stepped up screening measures for travelers from China, especially those arriving from Wuhan. North Korea has banned foreign tourists, a step it also took during the SARS outbreak and in recent years due to Ebola.

China has been credited for responding rapidly to this outbreak, in stark contrast to how it withheld information for months on SARS, allowing the virus to spread worldwide.

One veteran of the SARS outbreak said that while there are some similarities in the new virus---namely its origins in China and the link to animals--the current outbreak appears much milder.

Dr. David Heymann, who headed WHO’s global response to SARS in 2003, said the new virus appears dangerous for older people with other health conditions, but doesn’t seem nearly as infectious as SARS.

“It looks like it doesn’t transmit through the air very easily and probably transmits through close contact,” he said. “That was not the case with SARS.”