Dead chicken found at a Hong Kong farm have tested positive for the H5 strain of the avian influenza virus, which were the first confirmed bird flu cases on a local farm in about six years, health authority said Tuesday.

"Three dead chicken found at a chicken farm at Yuen Long district were H5 positive after our laboratory tests," York Chow, Secretary for Food and Health of the Hong Kong Special Administrative Region (HKSAR) government, told a press conference.

The alert level for bird flu has been raised to level "serious", he added.

Chow said two chicken farms in the areas have been closed down following the H5 cases and the HKSAR government has declared the areas within 3 kilometers of the affected farm an infected place and ordered the culling of about 80,000 chickens on the two chicken farms as well as on live poultry markets in the affected areas.

Health authority has decided to suspend the imports of live poultry to the city and the trading of live poultry on local markets for 21 days starting Tuesday.

"Those staff in close contact with live poultry in the farms are in good health now without any syndromes of H5 infection," Chow said, "but we will closely monitor the situation."

The Food and Health Bureau had put all local hospitals on high alert and require them to report timely the possible suspected human cases of bird flu infection.

"I also call on the public to observe personal hygiene," he added.

The Leisure and Cultural Services also announced on Tuesday the temporary closure of the Aviary Pagoda, or a bird watching site, in Yuen Long Park, which is within 3 km of the affected chicken farm as a precautionary measure.

Health staff have also been dispatched to clean certain local markets.

Chow said the local health authority has yet to make further tests to decide whether there has been any mutation in the strain of the bird flu virus involved.

The deadly H5N1 strain of the bird flu virus has killed many people worldwide over the past decade, in addition to leading to the culling of millions of domestic birds. Scientists worry that a mutation in the virus could enable its transmission from human to human, thereby leading to pandemic that could potentially kill millions.

Hong Kong has recently seen cold and dry weather conditions, in which the bird flu virus is typically more active, an epidemiologist at the University of Hong Kong said, adding that the risks of the bird flu cases spreading to retail markets were low.

Hong Kong has been on high alert against a potential bird flu outbreak in the past years, with precautionary measures and an alert system that responds quickly.

It has been trying to retrieve some of the live chicken trading licenses over the past months and centralize the processing of live chicken before they hit the market.

Editor: Yan