BEIJING — Chinese health authorities are trying to lock down Wuhan, the metropolis of 11 million people that is at the heart of a spreading coronavirus outbreak, in an extraordinary effort to stop new infections during the busiest travel period of the year.

All outbound trains and bus services from Wuhan — larger than any city in the United States — were suspended starting at 10 a.m. Thursday, causing chaos for some of the 400 million people hitting the road for the Lunar New Year holiday, which officially begins Friday.

Armed police guarded the entrance to Wuhan's biggest railway station, less than a mile from the market where the virus originated, to stop people trying to get onto the last trains out of the city. "Whatever train ticket I can get, as long as I can get out of Wuhan," one would-be passenger at Han­kou station told a local reporter early Thursday.

Health workers in hazmat suits checked the temperatures of passengers driving out of the city, while 200 of the 600-odd flights scheduled at Wuhan's international airport were canceled.

"Unless there are special reasons, citizens should not leave Wuhan," the new Coronavirus Infection Pneumonia Epidemic Prevention and Control Headquarters in Wuhan said in a notice announcing the travel ban.

At least 17 people have died, all of them in Hubei province, of which Wuhan is the capital, since the coronavirus broke out at the end of last month. More than 570 people nationwide, almost all of them in Hubei, have been confirmed as infected.

A handful of cases have been reported abroad, including in the United States, Japan and South Korea, but the World Health Organization (WHO) postponed a decision Wednesday on whether to declare a public health emergency.

Chinese health officials said they had ascertained that the virus started in an unsanitary food market that was selling wild and exotic animals for consumption. Snakes were the most likely cause of the virus, five Chinese scientists concluded in a paper published Wednesday in the Journal of Medical Virology.

It was unclear whether China’s ramped-up response would actually halt the spread of the virus, and it drew mixed reactions from health experts.

“In past outbreaks, we haven’t seen evidence that large-scale quarantine diminishes spread of disease,” said Tom Inglesby, ­director of the Center for Health Security at the Johns Hopkins Bloomberg School of Public Health. “It runs the risk of people losing confidence in government. And it places enormous responsibilities on government to make sure that people can continue to get food, basic necessities, medicines.”

The new restrictions could also make people more reluctant to report illness and might encourage them to escape the area of the quarantine, he said. “I would focus on isolating the people who are sick.”

But WHO Director General Tedros Adhanom Ghebreyesus praised the decision to shutter one of China’s major transportation hubs as “a very strong measure” that would reduce mass gatherings that often spread infection.

“Taking the action they think is appropriate is very important,” he said in a news briefing Wednesday in Geneva. He added that as long as the response is “tailored to the problem and the commitment we see, we commend their actions.”

The agency’s team in Wuhan would assess the impact on the ground, he said.

Chinese state media struggled to keep its tally up to date Wednesday as new cases continued to be reported across the length and breadth of the country. The Chinese territories of Hong Kong and Macao also confirmed their first infections, with Hong Kong reporting a second case early Thursday. Macao said it would cancel public celebrations planned to mark the new year this weekend.

The emergence of a new illness out of China has carried echoes of the severe acute respiratory syndrome (SARS) epidemic, which sparked mass panic as it spread across more than two dozen countries over eight months after starting in China in November 2002. SARS caused an estimated $30 billion in economic losses, experts have said.

But with SARS, it took months to identify the cause — civet cats, a delicacy in southern China. In contrast, the virus in Wuhan was identified quickly and its genome sequenced and shared within days. Chinese health officials have confirmed that the pneumonia-like virus came from animals sold at a live-animal market in Wuhan.

Members of the Geneva-based WHO said in a briefing Wednesday that they were split on whether to issue an emergency declaration after listening to presentations from Chinese officials and others. The committee made recommendations to contain the virus both in China and other countries, and plans to meet again on Thursday, Tedros said.

“The decision of whether to declare a public health emergency is one I take extremely seriously and one I am only prepared to make with appropriate consideration of all the evidence,” he said. “We will have much more tomorrow.”

The two factors determining such a designation are severity of illness and the extent and nature of the disease transmission, said Didier Houssin, a Russian ­infectious-disease expert who chairs the WHO’s emergency committee on the coronavirus.

“The committee felt it was a little too imprecise to suggest to [Tedros] to declare” an international public health emergency, Houssin said.

Officials said it’s not unusual in outbreaks of respiratory illness for patients to become infected through close contact with family members and in health-care settings, as has happened in Wuhan. What matters is the route of transmission and whether authorities can track those contacts down, they said.

“There are still many unknowns,” said Michael Ryan, the WHO’s executive director of health emergencies.

He said that the patients who died in China tended to be older people, and that 40 percent had significant underlying medical conditions, a feature of previous coronavirus outbreaks.

He noted that all viruses change but that this one is showing stability. “We’re not seeing huge changes in the viral genomic sequence,” he said.

President Trump said from Davos, Switzerland, where he was attending the World Economic Forum, that Americans have nothing to worry about because there was only a single case in the United States so far. “It’s one person coming in from China, and we have it under control,” he said.

But at least so far, the virus has defied control in China.

“At present, during the Spring Festival travel rush, the number of people on the move has surged, which has increased the risk of an epidemic and the difficulty of preventing and controlling transmission,” National Health Commission Vice Director Li Bin said Wednesday, using the Chinese name for the holiday.

Highlighting the difficulties of the response effort, Wang Guangfa, a member of the National Health Commission’s expert group that traveled to Wuhan to investigate the outbreak, was among the infected. He wrote in a social media post that he thought he contracted the virus through his eyes because he had been wearing a mask. But, he said, he was responding well to treatment.

The outbreak could hardly come at a worse time. Before the travel suspension in Wuhan, officials estimated that 3 billion trips would be made in China during the 40-day period around the turn of the Lunar New Year. It is a time notorious even in China for crowded trains and buses, conditions that are ripe for the airborne virus to spread.

Authorities had already mandated use of medical masks in public places. Across China, people raced to buy specific masks that can protect against the microbes. Many physical and online stores had sold out on Wednesday, leading Taobao, the huge online marketplace, to ban its online vendors from raising mask prices to exploit the crisis.

One mask manufacturer in Zhejiang province said it received orders for 80 million masks in the space of a week and has had to call 100 workers back from their holidays and promise to pay them triple their usual salary. Still, the company has said it will not be able to meet demand.

“We should not take this situation lightly and should be on high alert,” Li said Wednesday in a news conference called by central government authorities to allay concerns after confirmation that the virus could be spread between humans.

Wuhan, which straddles the banks of the Yangtze River, is a major transport hub for trains, planes and ships.

Some 27 million passengers traveled through Wuhan Tianhe International Airport last year. The airport is among those that offer six-day visa-free entry into China, part of an effort to encourage foreign tourists to stop over on their way to other countries. International destinations that are reachable by direct flights from Wuhan include New York, San Francisco, London, Paris and Moscow.

Planes bound for Anchorage, Rome and Tokyo took off as usual Thursday morning.

The airport in Wuhan is also one of the busiest for domestic aviation, sitting at the intersection of the Beijing-Guangzhou and ­Shanghai-Chengdu air corridors. China Southern, the country’s biggest airline, canceled all flights out of Wuhan, but smaller carriers continued operating many domestic routes.

With an expansion completed this month, the number of flights in and out of the airport had been expected to increase from 600 to 800 a day during the Spring Festival travel rush.

At the same time, China Railway’s Wuhan branch had been expecting almost 25 million passengers to travel through its three main stations this Spring Festival, with a daily peak of about 920,000 rail passengers.

Sun reported from Washington. Yasmeen Abutaleb in Washington and Liu Yang and Lyric Li in Beijing contributed to this report.