HONG KONG (AP) - The number of patients in Hong Kong who are affected by the still mysterious Wuhan flu jumped to 15 on Sunday (Jan 5).

The overnight increase of seven more patients has prompted the city's authorities to activate a newly created "serious response" level as fears spread that the infectious disease might have been imported there by visitors to Wuhan.

At least 59 people in Wuhan, an inland city west of Shanghai about 900km north of Hong Kong, are being treated for the viral pneumonia-like illness.

Hong Kong Secretary for Food and Health Sophia Chan Siu Chee said they had been in close contact with the mainland authorities for the latest updates. "People are also reminded not to visit the wet market or eat wild animals when travelling," she said.

The latest patients are aged between 22 and 55. The South China Morning Post reported that all the patients were in stable condition and were being quarantined in four different hospitals in the city.

One of them was confirmed to have the flu.

The outbreak, which emerged last week, has revived memories of the Sars epidemic in 2003 that started in southern China and killed more than 700 people in the mainland, Hong Kong and elsewhere.

The serious response level indicates a moderate impact on Hong Kong's population of 7.5 million people. It is the second-highest in a three-tier system that is part of a new government plan to respond to infectious diseases of unknown cause.

The city's health department has put more thermal imaging cameras at Hong Kong's airport to check the body temperature of arriving passengers. More staff have also been assigned for temperature checks at the West Kowloon high-speed rail station that connects Hong Kong to the mainland.

Meanwhile Chinese healthcare authorities in Wuhan confirmed that the illness was not Severe Acute Respiratory Syndrome (SARS), Middle East respiratory syndrome (MERS) or bird flu, even though they were still working to identify the cause and source. Of the 59 patients so far, seven are still in critical conditions while 163 are under observation for having contacts with the other patients.

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Most of the cases were traced to the South China Seafood City food market in the suburbs of sprawling Wuhan, where offerings reportedly include wild animals that can carry viruses dangerous to humans.

The commission said the market has been disinfected. The most common symptom has been fever, with shortness of breath and lung infections in a small number of cases.

There have been no clear indications of human-to-human transmission of the disease. Besides Sars, Hong Kong was also hit by bird flu in 1997 and swine flu in 2009.