Health chiefs in China are investigating an outbreak of a respiratory illness that some people have likened to the 2003 Sars epidemic that killed nearly 800 people.

Doctors say 27 people have fallen ill in December with a suspected strain of viral pneumonia, seven of whom are in serious condition.

Most of the sick had visited a seafood market in the central city of Wuhan, and experts are investigating whether the disease is linked to it.

“The cause of the disease is not clear,” the official People’s Daily newspaper posted online, citing hospital officials. “We cannot confirm it is what’s being spread online, that it is the Sars virus. Other severe pneumonia is more likely.”

Experts from the National Health Commission travelled to Wuhan to lead the investigation into the disease, state television reported.

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Doctors have yet to identify the virus responsible but initial laboratory tests showed that the cases were viral pneumonia and had not spread from person to person, according to the Wuhan health commission.

Patients were isolated and their close contacts were under medical observation. An investigation and clean-up were under way at the seafood market, the commission said.

Two of the 27 had improved so much they were expected to leave hospital soon, the health authorities added.

Social media users suggested the outbreak could be linked to Sars, which emerged in southern China in late 2002, spreading rapidly to other cities and countries.

More than 8,000 people were infected worldwide and 775 died, according to the World Health Organisation.

Chinese authorities covered up the outbreak for weeks before a growing death toll and rumours forced the government to reveal the epidemic, apologise and vow full openness in future outbreaks.

One man in Britain who was in contact with a Hong Kong businessman was the first person in the UK to become infected.