BEIJING — Chinese health ­authorities sought to impose a ­quasi-quarantine Tuesday around the hot spot of a mystery pneumonia-like virus that has now claimed nine lives in China and was confirmed in the United States for the first time.

The U.S. case — a man in his 30s under observation in Washington state — had links to the area of most concern in China: the commercial center of Wuhan about halfway between Beijing and Hong Kong.

In an attempt to contain the virus, Chinese authorities advised people in the city of 11 million not to leave. But the U.S. case showed how far the virus has moved beyond the Wuhan region.

U.S. officials said the man, a resident of Snohomish County, Wash., returned Jan. 15 from a trip to the region around Wuhan. Shortly after arriving at Seattle’s international airport, he began feeling ill and got in touch with his health-care provider on Sunday.

The Centers for Disease Control and Prevention confirmed Monday that the man had the coronavirus that has sickened more than 400 people in China and others in Thailand, Japan, Taiwan and South Korea.

The Geneva-based World Health Organization said it would hold an emergency meeting Wednesday to decide whether to designate the outbreak as an international public health emergency.

The man was in stable condition at Providence Regional Medical Center in Everett, Wash. Officials said they are monitoring him there out of an abundance of caution, not because he is seriously ill.

CDC officials said they were expanding screenings to international airports in Atlanta and Chicago. Measures were already in place at the international airports in Los Angeles and San Francisco, and at New York’s John F. Kennedy International, the first such effort since the Ebola outbreak in 2014.

Federal officials will also direct travelers arriving in the United States on direct and indirect flights from Wuhan to those five airports for screening. That process is being worked out in the coming days. For example, if a passenger was originally to fly from Wuhan to Shanghai and then Boston, that flight would most likely be rerouted to JFK for screening, and then proceed to Boston, CDC officials said.

But the Washington state man arrived before the airport screenings began.

“This is an evolving situation, and again, we do expect additional cases in the United States and globally,” said Nancy Messonnier, the director of CDC’s National Center for Immunization and Respiratory Diseases.

In China, nine people, all of them in Wuhan, have now died from the virus, Li Bin, vice director of the National Health Commission, told reporters on Wednesday morning.

“The virus has the potential to mutate and there is a risk it could further spread,” he said, adding that authorities were on “high alert.”

A total of 440 people have been confirmed as having the infection, covering the length of the country. One case has been discovered in the territory of Macao and there are 117 suspected cases in Hong Kong, where authorities have banned the wearing of face masks as a result of recent protests.

With confirmation that the coronavirus could be transmitted person to person and wth hundreds of millions of people packing onto public transport to make their annual visits home for the Lunar New Year, a new sense of panic has set in.

Long lines formed at pharmacies and convenience stores around the country as people rushed to buy surgical masks, with unlucky customers posting photos on social media of bare shelves. People around the country canceled their trips home for the Spring Festival, as new year celebrations are known, the most important holiday on the Chinese calendar.

A deadly coronavirus has sickened more than 4,565 in China as officials there fear a broad quarantine isn't enough to stem the virus. (The Washington Post)

“I don’t really dare to go to the airport right now, or even to the movie theater,” said Xie Jing, 33, who works in advertising in Shanghai, where two cases of the coronavirus have been confirmed. She canceled her planned trip home to Sichuan, where two cases are suspected.

“Everyone is being very careful at the moment in Shanghai. Everyone is wearing masks on the streets,” Xie said.

The virus was first detected Dec. 31 and was linked to a dirty food market in Wuhan, not far from one of the city’s main train stations, where wild animals including wolf pups and civet cats had been on sale for consumption.

The Washington state man did not visit the Wuhan market and did not know anyone who was ill, U.S. officials said. The man was visiting relatives in the Wuhan area, U.S. officials said.

Initially, doctors thought the virus was not communicable between humans, but cases of infection across the country, including among people who have not been to Wuhan, indicated person-to-person infection.

Specialists are advising against travel in and out of Wuhan, which is in east-central China.

“We hope people can avoid going to Wuhan if possible and that people in Wuhan can stay there,” said Zeng Guang, the chief epidemiologist at the Chinese Center for Disease Control and Prevention and the leader of a government team of experts responding to the outbreak. “This is not a call from the officials but a suggestion from us in the expert team,” he said.

Still, Zeng said it was “inevitable” that the virus would continue to spread as people moved around the country for the Lunar New Year on Saturday.

The Ministry of Transport estimates that 400 million people will be on the move, making a total of 3 billion trips in this period.

Health authorities added more infrared thermometers at Wuhan airport and train stations to check passengers for fever, and some hotels in the city also began requiring that temperatures be taken before customers could check in. Outbound group tours have been restricted.

Traffic police began conducting random checks on vehicles traveling in and out of the city to ensure they were not transporting live birds or wild animals.

Some airlines and travel agencies began offering refunds to people traveling out of Wuhan or people with the virus, and some hotels have allowed people to cancel their reservations without penalty. At least two airlines flying to Wuhan have stocked their planes with masks.

Wuhan’s three major hospitals have 800 beds, but authorities said they would add 1,200 beds to deal with the rising number of pneumonia cases. They also said they would foot the hospital bills for those infected.

The measures come after criticism that Wuhan authorities have been lax in stopping the spread of the virus.

On Saturday, as the virus exploded in Wuhan, the city held potluck banquets to celebrate the looming holiday, with more than 40,000 families attending. News and photos of the event appeared Sunday on the front page of the state-run newspaper in Wuhan, but they were deleted from the Internet by Tuesday amid criticism about the lack of health precautions.

The city had still planned to go ahead with 41 large-scale events for holiday celebrations, advertising them on Monday, but it announced Tuesday that they have been “postponed.” Schools and universities are on break for Spring Festival, but more than 100 extracurricular “cram” schools in Wuhan have canceled classes.

Quarantine was the most effective way to prevent transmission of the virus, since it spreads by droplets from the nose and mouth, said Zhong Nanshan, the leader of a group of experts at China’s National Health Commission.

“Now our big concern is if a super spreader emerges,” Zhong said Tuesday at a news conference in the southern province of Guangdong, using the term for a carrier who infects a disproportionately high number of people. A “super spreader” is thought to have passed the virus on to 15 medical workers at a Wuhan hospital.

Although some hospitals have been stockpiling antibiotics, they are not effective against viruses. “There’s no specific drug to treat the infection at the moment,” Zhong said.

The Chinese CDC said the latest virus is the seventh type of coronavirus known to affect humans. The previously known six viruses include Severe Acute Respiratory Syndrome (SARS) and Middle East respiratory syndrome (MERS), which are linked to animals.

Chinese health authorities have added the new type of pneumonia to the Class B list of infectious diseases, in the same category as SARS and HIV. But they said they would enforce the strictest controls, usually used for the most dangerous illnesses, Class A diseases such as cholera and the plague, to try to contain the coronavirus.

That meant authorities could forcibly quarantine people known to have or suspected to have the coronavirus, and would inform the public of every new case nationwide. Immigration authorities have also added the new pneumonia to a list of infectious diseases of interest to them.

The Foreign Ministry in Beijing said that there was not a complete ban on movement in and out of Wuhan.

“The Wuhan government has already taken measures to control the flow of people leaving Wuhan,” said Geng Shuang, a spokesman. “I understand when they are leaving or when they are entering, there will be checks, but there’s not a complete ban of all people leaving.”

The government was sharply criticized as having played down or covered up the extent of the SARS virus, but experts say Chinese authorities have learned many lessons in the 17 years since then.

“The new pneumonia in Wuhan reminds many people of the SARS epidemic in 2003,” said a social media account run by the Central Political and Legal Affairs Commission, vowing not to repeat those mistakes.

“Self-deception will only make the epidemic worse and turn a natural disaster that was controllable into a man-made disaster at great cost,” said the post, which was later deleted. “Only openness can minimize panic to the greatest extent.”

Fortuitously, Wuhan is home to the highest-grade biosafety laboratory in China, a level-four facility that opened two years ago and is designed for work on the most dangerous microbes, such as Ebola and Lassa fever viruses.

When it opened, the lab was hailed as a “significant breakthrough” in building China’s public health defense system, with state media calling it an “aircraft carrier” for virus research and a facility that provided “firewall virus protection” for the country of 1.4 billion people.

Sun and Bernstein reported from Washington. Lyric Li, Liu Yang and Wang Yuan contributed to this report.