The number of people infected with a new virus in China tripled over the weekend, with the outbreak spreading from Wuhan to other major cities.
There are now more than 200 cases, mostly in Wuhan, though the respiratory illness has also been detected in Beijing, Shanghai and Shenzhen.
Three people have died. Japan, Thailand and South Korea have reported cases.
The sharp rise comes as millions of Chinese prepare to travel for the Lunar New Year holidays.
Health officials have identified the infection, which first appeared in Wuhan in December, as a strain of coronavirus. They say it has led to an outbreak of viral pneumonia, but much about it remains unknown.
Although the outbreak is believed to have originated from a market, officials and scientists are yet to determine exactly how it has been spreading.
The outbreak has revived memories of the Sars virus – also a coronavirus – that killed 774 people in the early 2000s across dozens of countries, mostly in Asia. Analysis of the genetic code of the new virus shows it is more closely related to Sars than any other human coronavirus.
Experts in the UK told the BBC the number of people infected could still be far greater than official figures suggest, with estimates closer to 1,700.