Hong Kong raised its bird flu alert level to "serious" on Tuesday and announced it is to cull 17,000 chickens after three birds tested positive for the deadly H5N1 strain of the virus.
The city's health chief York Chow announced the measures after a dead chicken at a wholesale market and two other wild birds tested positive for the deadly H5N1 strain of the disease.
Authorities banned all live poultry imports with immediate effect while they trace the source of the dead chicken, whether it was imported or from local poultry farms.
Public broadcaster RTHK reported that around 20 students from a girls' school, aged between six and seven, have developed flu-like symptoms including fever, cough and sore throat, but none has been taken to hospital.
Hong Kong was the site of the world's first major outbreak of bird flu among humans in 1997, when six people died from a mutated form of the virus, which is normally confined to poultry. Millions of birds were then culled.
The cull at the poultry wholesale market where the infected chicken was found will be held on Wednesday, Chow told a late night press conference.
"With a heavy heart, I announce that the dead chicken has been tested positive for the H5N1 strain of virus after a routine check by the agriculture, fisheries and conservation department today," Chow said.
"We are now raising the bird flu response level from alert to serious."
Earlier on Tuesday, the agriculture authorities confirmed an Oriental magpie robin found dead in a secondary school on Saturday had tested positive for H5N1, the second such case in a week.
Another secondary school was ordered to close for a day for disinfection last Friday after a dead black-headed gull was found with the virus.
A school clerk, who picked up the bird, and her son developed flu-like symptoms and were taken to hospital but both were cleared later.
A 59-year-old woman tested positive for bird flu in 2010 in Hong Kong's first human case of the illness since 2003.
The city is particularly nervous about infectious diseases after an outbreak of deadly respiratory disease SARS in 2003 killed 300 people in Hong Kong and a further 500 worldwide.