A new virus that has killed nine people could mutate and spread further, Chinese health officials have warned, as they step up containment measures.
There are 440 confirmed cases, with the origin a seafood market that “conducted illegal transactions of wild animals”.
It has now spread from Wuhan to several Chinese provinces as well as the US, Thailand and South Korea.
Authorities admitted that the country was now at the “most critical stage” of prevention and control.
China had earlier confirmed that human-to-human transmission of the virus had taken place.
The virus, known also as 2019-nCoV, is understood to be a new strain of coronavirus that has not previously been identified in humans.
Signs of infection include respiratory symptoms, fever, cough, shortness of breath and breathing difficulties.
In one of the first public briefings since the beginning of the outbreak, National Health Commission vice-minister Li Bin said there was evidence that the disease was “mainly transmitted through the respiratory tract”.
But China has still not been able 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,” said Mr Li.
He also added that there were 2,197 people who were known to have come into contact with infected patients.
No super spreader – a patient who has transmitted the virus to more than 10 people – has been discovered so far.
At least 15 medical workers in Wuhan, presumably those who came into contact with patients, are known to be infected with the virus.