Chinese authorities have urged people to stop travelling in and out of Wuhan, the city at the centre of a new virus outbreak that has killed nine people.
Those living in the city of 8.9 million people have also been told to avoid crowds and minimise public gatherings.
The new virus has spread from Wuhan to several Chinese provinces, as well as the US, Thailand and South Korea.
There are 440 confirmed cases, with the origin a seafood market that “conducted illegal transactions of wild animals”.
“Basically, do not go to Wuhan. And those in Wuhan please do not leave the city,” said National Health Commission vice-minister Li Bin in one of the first public briefings since the beginning of the outbreak.
Authorities also admitted that the country was now at the “most critical stage” of prevention and control.
Meanwhile, in Geneva, the World Health Organization’s emergency committee is meeting to assess the global risks posed by the virus and decide if it should be declared an international public health emergency – as happened with swine flu and Ebola.
Such a declaration, if made, could see advice issued on travel or trade restrictions.
Earlier this week, China 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. The Sars virus that killed nearly 800 people globally in the early 2000s was also a coronavirus.
Signs of infection with the new virus include respiratory symptoms, fever, cough, shortness of breath and breathing difficulties.
The first US case was confirmed on Tuesday. President Donald Trump said the situation was “totally under control” and that he trusted the information being provided by Chinese authorities.
Mr Li said there was evidence that the disease was “mainly transmitted through the respiratory tract”. In general, coughs and sneezes are a highly effective way for viruses to spread.
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 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, who presumably came into contact with patients, are known to be infected.