China expanded its lockdown against the deadly new virus to an unprecedented 36 million people - and is rushing to build a prefabricated, 1000-bed hospital for victims.

The number of confirmed cases around the world climbed to more than 850, with at least 25 deaths, all of them in China.

The US reported its second case, involving a Chicago woman in her 60s who was hospitalised after returning from China. She was reported to be doing well.

Transportation was shut down in Wuhan, the city of 11 million at the epicentre of the outbreak, and in at least 12 other cities in central China's Hubei province, encompassing a population bigger than that of New York, London, Paris and Moscow combined.

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Heavy equipment works at a construction site for a field hospital in Wuhan. Photo / AP
Heavy equipment works at a construction site for a field hospital in Wuhan. Photo / AP

Authorities in Beijing and other cities cancelled many public celebrations and other events marking Lunar New Year, which falls on Saturday.

Hospitals in Wuhan grappled with a flood of patients and a lack of supplies. Videos circulating online showed throngs of frantic people in masks lined up for checks, and some people complained that family members who had sought diagnoses were turned away at hospitals that were at capacity.

Authorities in Wuhan and elsewhere put out calls for medicine, disinfection equipment, masks, goggles, gowns and other protective gear.

Wuhan authorities said they are rapidly constructing a new hospital to deal with the crisis, to be completed February 3.

It will be modelled on a SARS hospital that was built in Beijing in just six days during the SARS outbreak.

The seriousness of the crisis was still an open question.

It was not clear just how lethal the virus is, or even whether it is as dangerous as ordinary flu, which kills tens of thousands of people in US alone every year.

Scientists say it is also not clear if it spreads as easily as SARS, its genetic cousin, which also originated in China and killed about 800 people in 2002-03.

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The rapid increase in reported deaths and illnesses does not necessarily mean the crisis is getting worse.

It could instead reflect better monitoring and reporting of the newly discovered virus, whose symptoms can initially resemble those of the cold and flu, including cough, fever and shortness of breath, but can worsen to pneumonia.

"It's still too early to draw conclusions about how severe the virus is because at the beginning of any outbreak you would focus more on the severe cases," said Tarik Jasarevic, a spokesman for the World Health Organisation in Geneva.

"And then maybe we are missing some mild cases because people will just be a little bit sick and will not have it tested. And they will recover."

With authorities afraid that public gatherings will only worsen the spread of the virus, temples locked their doors, Beijing's Forbidden City, Shanghai Disneyland and other major tourist destinations closed, and people cancelled restaurant reservations ahead of the most important holiday on the Chinese calendar, normally a time of family reunions and sightseeing trips in the country of 1.4 billion people.

Peng Zhiyong, centre, head of the department of critical care medicine, and his colleagues attend to a patient at Zhongnan Hospital in Wuhan. Photo / AP
Peng Zhiyong, centre, head of the department of critical care medicine, and his colleagues attend to a patient at Zhongnan Hospital in Wuhan. Photo / AP

Wuhan's usually bustling streets, malls and other public spaces were unnervingly quiet on day two of its lockdown, and masks were mandatory in public.

Shoppers emptied store shelves, stocking up for what could be an extended period of isolation. Karaoke bars, movie theatres and internet cafes around the region were shut down.

While most of the deaths have been older patients, a 36-year-old man in Hubei died on Thursday.

The vast majority of cases have been in and around Wuhan or involved people who visited the city or had personal connections to those infected.

Fewer than two dozen cases in all have been confirmed outside mainland China, in Hong Kong, Macao, the US, South Korea, Japan, Singapore, Thailand, Taiwan and Vietnam. Many countries are screening travellers from China and isolating anyone with symptoms.

Chinese officials have not said how long the shutdowns of the cities will last.

Recalling the government's initial cover-up of SARS, many Chinese are suspicious of the case numbers reported by officials. Authorities in turn have promised transparency.

China's cabinet, the State Council, announced it will be collecting information on government departments that have failed in their response to the outbreak, including "delays, concealment and under-reporting of the epidemic".

The state broadcaster CCTV's annual Spring Festival Gala programme, which attracted more than 1 billion viewers last year, paid tribute to the medical workers fighting the viral outbreak.

Heavy equipment at a construction site for a field hospital in Wuhan. China is building a 1000-bed coronavirus hospital, scheduled to be open on February 3. Photo / AP
Heavy equipment at a construction site for a field hospital in Wuhan. China is building a 1000-bed coronavirus hospital, scheduled to be open on February 3. Photo / AP

"Please believe in China," the hosts said. "With the most transparent public information ... on the battlefront of the epidemic, we will definitely win."