Chinese authorities have launched an investigation into a mysterious viral pneumonia which has infected dozens of people in the central city of Wuhan.
A total of 44 cases have been confirmed so far, 11 of which are considered "severe", officials said on Friday.
The outbreak has prompted Singapore and Hong Kong to bring in screening processes for travellers from the city.
It comes amid online fears the virus could be linked to Sars, or severe acute respiratory syndrome.
The potentially deadly, flu-like Sars virus killed more than 700 people around the world in 2002-03, after originating in China.
There has been speculation on social media about a possible connection to the highly contagious disease.
Wuhan police said eight people had been punished for "publishing or forwarding false information on the internet without verification".
The Wuhan health commission said on Friday it was investigating the cause of the outbreak.
In a statement on its website, it said it had already ruled out a number of infection sources - including influenza, avian influenza and common respiratory diseases - but did not mention Sars.
There has also been no human-to-human transmission, the statement added. However, a number of those infected worked at a seafood market in the city, leading authorities to clean the area.
A spokesman for the World Health Organization (WHO) said it was aware of the outbreak and was in contact with the Chinese government.
"There are many potential causes of viral pneumonia, many of which are more common than severe acute respiratory syndrome coronovirus," the spokesman added. "WHO is closely monitoring this event and will share more details as we have them. "
Fears sparked by an older epidemic
Analysis by BBC Health's Philippa Roxby
This latest outbreak appears to have sparked memories for those who dealt with a Sars epidemic 18 years ago.
At the time, the WHO criticised China for under-reporting the number of cases of Sars in a southern Chinese province.
In the 2002-03 epidemic, the virus affected more than 8,000 people in 26 countries, killing 349 people in mainland China and 299 in Hong Kong.
Travellers flying to other countries are thought to have been behind the large number of cases in that outbreak because Sars spreads quickly without swift treatment in hospital.
China sacked its health minister at the time for the poor handling of the crisis.
The country has been free of Sars since May 2004.