The coronavirus is a large family of viruses that causes illnesses ranging from the common cold to acute respiratory syndromes, but the virus that has killed people in China is a novel strain and not seen before.
Common symptoms of the novel coronavirus strain include respiratory symptoms such as fever, cough, shortness of breath and breathing difficulties, according to the WHO.
The Chinese health authorities said that 444 confirmed cases of pneumonia caused by a novel coronavirus (2019-nCoV) had been reported in 13 provincial-level regions in the country.
Considering its potential to spread to different countries, the World Health Organisation (WHO) is holding a rare emergency meeting Geneva on Wednesday to consider declaring Wuhan virus as an international public health emergency, as it did with swine flu and Ebola.
Such a declaration, if made, will be seen as an urgent call for a coordinated international response in view of fears that millions of Chinese are travelling at home and abroad for the Chinese New Year and Spring Festival holidays starting from January 24.
The festival triggers biggest mass migration, clogging road, rail and air networks every year.
Altogether 149 new confirmed cases were reported on Tuesday, Li Bin, deputy director of the National Health Commission, told at a press conference here on Wednesday.
Li said China was now at the "most critical stage" of prevention and control following confirmation that that human-to-human transmission of the virus had taken place.
The official said there was evidence that the disease was "mainly transmitted through the respiratory tract".
With most cases outside Wuhan are linked to the city, Chinese officials are advising 11.5 million people of the city not to leave to make it easier to contain the virus.
"Basically, do not go to Wuhan. And those in Wuhan please do not leave the city," Li said.
State-run Xinhua news agency reported that strict measures have been taken to prevent the spread of the virus. A total of 15 thermal detectors have been set up in the Wuhan Tianhe International Airport, and 20 thermal detectors have been installed at three major railway stations of the city.
The city will also strictly monitor and control the farmers' markets, supermarkets and restaurants as well as toughen crackdowns on wild animal trade, Li said.
He added that public gatherings will be restricted, and unnecessary entry and exit trips to Wuhan will be called off.
Officials also cancelled Friday prayers of Muslims in mosques in the city.
"We should be strictly on guard against the pneumonia situation spreading outside Wuhan," Li said.
Gao Fu, head of the Chinese Centre for Disease Control and Prevention, said no evidence has so far emerged showing that a super-spreader, or a highly contagious virus carrier.
He added that based on current evidence, the virus was originated from wild animals sold at a seafood market of Wuhan.
In accordance with laws and regulations, quarantine measures are being taken on entry and exit ports, Li said.
Overseas, one case has been confirmed in Japan, three in Thailand, and one in Korea. The first case of the virus has now been reported in Seattle in the US with official confirmation that a person arrived from Wuhan.
The patient was hospitalised on Sunday, after arriving in Seattle from Wuhan on January 15, officials at the US Centres for Disease Control and Prevention (CDC) said.
Latest cases have been reported from Hong Kong, Macao and Mexico. Many Chinese cities including Beijing have already reported such cases.
Li said the figures from the commission show a total of 2,197 close contacts have been traced. A total of 1,394 are under medical observation while 765 people have been discharged.
Most of the deaths were reported in central Hubei province, where the virus first appeared in the city of Wuhan.
China is yet to confirm the exact source of the virus.
"Though the transmission route of the virus is yet to be fully understood, there is a possibility of virus mutation and a risk of further spread of the epidemic," Li said.
The Wuhan Health Commission said it would authorise medical institutions to quarantine all patients and people in close contact with them as a prevention and control measure, Li said.
On Tuesday, the National Health Commission upgraded the coronavirus to a Class B infectious disease, but said it would use the stricter control measures for a Class A disease to handle the outbreak, meaning any infection nationwide must be reported within two hours and monitored.
India has already issued a travel warning and monitoring the situation as Wuhan has about 700 Indian students.