The new virus that has killed nine people in China is mutating and could spread further, health officials said.
The warning came as 440 cases of the coronavirus were confirmed on Wednesday.
Another 2,197 cases of close contact with patients had been confirmed and there was evidence of "respiratory transmission" of the virus, National Health Commission vice-minister Li Bin told reporters.
Fifteen medical personnel are among those infected in China, with symptoms including fever, coughing and difficulty breathing.
The viral infection can cause pneumonia and can be passed from person to person.
Though the origin of the virus has yet to be identified, the World Health Organisation (WHO) said the primary source was probably animal.
The virus originated in the central city of Wuhan at the end of last year and has since spread to Beijing to Shanghai.
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Cases have also been found in the US, Thailand, South Korea, Japan and Taiwan.
The autonomous region of Macau recorded its first case of the coronavirus on Wednesday.
Chinese officials have linked the outbreak to Wuhan's seafood market.
China has stepped up efforts to control the outbreak by discouraging public gatherings in Hubei province, and people across the country were urged to avoid densely populated areas in general, the health commission said.
The WHO is due to hold an emergency meeting on Wednesday to determine whether the outbreak of the new coronavirus constitutes a global health emergency.
It came as China vowed to tighten containment measures in hospitals.
The Chinese government has been providing daily updates on the number of cases in a bid to head off public panic, as millions of people prepare to travel domestically and abroad for the country's lunar new year celebrations starting this week.
Li Bin has also said China will step up co-operation with the WHO.
Chinese Communist Party's Central Political and Legal Commission said in a post on its WeChat social media account that officials found to have covered up infections would be a "sinner for eternity before the party and the people".
The post was subsequently deleted.
Adam Kamradt-Scott, an infectious diseases expert at the Centre for International Security Studies at the University of Sydney, said: "I'm not sure that we could expect more of them at this stage in the outbreak, particularly when they are understandably focused on responding to the outbreak and trying to contain it ahead of the Chinese lunar new year celebrations."
Fears of a pandemic similar to the Severe Acute Respiratory Syndrome (SARS) outbreak that started in China and killed nearly 800 people between 2002 and 2003 have rocked global markets, with aviation and luxury goods stocks hit particularly hard and the Chinese yuan tumbling.
The WHO said on Tuesday the new coronavirus was likely to spread to other parts of China and possibly other countries in coming days.
WHO spokesman Tarik Jasarevic said new cases would appear as China stepped up monitoring.
He added: "If you increase surveillance and testing you are likely to get new numbers."
Hong Kong's Cathay Pacific Airways Ltd, one of the airlines most affected by the SARS outbreak, said it would allow flight attendants to wear a surgical mask while operating mainland China flights due to concerns over the new virus.
The Cathay Pacific Airways Flight Attendants Union said it had received a "tremendous" amount of messages from members concerned over catching the virus, and attendants on all flights should have the option to wear a mask.
The union said on its Facebook page: "All of them are worried about the risk they are taking every time they go to work."
Taiwan joined Australia in warning citizens to avoid travel to Wuhan, and airports around the world have stepped up screening of travellers from China.
Taiwan President Tsai Ing-wen wrote on her Facebook page: "I want to call on our nationals please not to visit this region if not necessary."