"The gray heron tested positive for H5N1," a government spokesman told Reuters, referring to the bird recently found dead in the Lok Ma Chau area near the mainland China border.

"We have already inspected the chicken farms nearby and we have found no bird flu symptoms or abnormal deaths among those chickens," Deputy Director of Agriculture, Fisheries and Conservation, S.P. Lau, told a news conference.

Health experts fear the virus, which has killed 12 people in Thailand and 20 in Vietnam this year, could mutate into a form that could be transmitted between humans.

Hong Kong has been a major concern for health experts, who are worried about a comeback of bird flu because the territory is a favorite resting place for migratory birds, a natural reservoir of the virus during the winter months.

China has reported bird flu outbreaks among poultry this year and experts say the disease may be carried by migrating birds.

On Tuesday, the government said the heron was suspected to have fallen victim to a virus belonging to the H5 bird flu family, which includes the H5N1 variant but also strains not known to affect humans.

Millions of chickens have been slaughtered across Southeast Asia as governments battle to contain a virus that World Health Organization officials fear could, if unchecked, eventually mutate and trigger a human flu pandemic.

The avian influenza virus was first seen in humans in 1997 in Hong Kong, where it infected 18 people and killed six.