The mysterious respiratory illness that has infected dozens of people in a central Chinese city is not SARS, local authorities say.

The 2002-2003 SARS epidemic started in southern China and killed more than 700 people in mainland China, Hong Kong and elsewhere.

Fears of a SARS recurrence arose this month after a slate of patients were hospitalised with an unexplained viral pneumonia in Wuhan, the capital of Hubei province.

As of Sunday, 59 people were diagnosed with the condition and have been isolated while they receive treatment, according to the Wuhan Municipal Health Commission.

Seven were in a critical condition, while the rest were stable.

Chinese authorities informed WHO that they have ruled out a number of causes of the outbreak of #pneumonia in Wuhan City, 🇨🇳. The pathogen is not influenza, avian flu, adenovirus, SARS or MERS. Work continues to identify the cause. — World Health Organization (WHO) (@WHO) January 5, 2020

The commission said in a statement on Sunday that initial investigations have ruled out SARS - severe acute respiratory syndrome - as well as Middle East respiratory syndrome, influenza, bird flu and adenovirus.

The commission previously said the condition's most common symptom was fever, with shortness of breath and lung infections appearing in a "small number" of cases.

There were no clear indications of human-to-human transmission.

Several patients were working at the South China Seafood City food market in sprawling Wuhan's suburbs.

The commission said the market would be suspended and investigated.

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Hong Kong's Hospital Authority said on Sunday that a total of 15 patients in Hong Kong were being treated for symptoms including fever and respiratory infection after recent visits to Wuhan.

Hong Kong's health chief Sophia Chan said hospitals and doctors have been directed to report cases of fever in anyone who has travelled to Wuhan in the past 14 days.

The hospital authority said it has activated a "serious response" level to curb the spread of the infection.

Chan warned Hong Kong residents against visiting wet markets and eating wild game in mainland China.

The World Health Organisation said it was closely monitoring the situation and maintaining contact with Chinese authorities.

No travel or trade restrictions are necessary at this time, the WHO said.