Hong Kong: Hong Kong authorities activated a newly created "serious response" level on Saturday as fears spread about a mysterious infectious disease that may have been brought back by visitors to a mainland Chinese city.

Five possible cases have been reported of a viral pneumonia that has also infected at least 44 people in Wuhan, an inland city west of Shanghai and about 900 kilometres north of Hong Kong.

Chief Executive Carrie Lam, second from right, reviews the health surveillance measures at West Kowloon Station in Hong Kong on January 3. Credit:AP

The outbreak, which emerged last month, has revived memories of the 2002-03 SARS epidemic 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 launched on Saturday to respond to infectious diseases of unknown cause.