HONG KONG - More than 17,000 chickens started to be culled at a poultry market in Hong Kong Wednesday after a chicken carcass there tested positive for the H5N1 avian influenza virus.

York Chow, the city's secretary for Food and Health, raised the city's response level for bird flu to 'serious' from 'alert' after the discovery of the virus in the carcass.

Chow announced Tuesday that a chicken carcass sample taken from the Cheung Sha Wan Temporary Wholesale Poultry Market was tested positive of the highly pathogenic H5N1 avian influenza virus during regular surveillance of the Agriculture, Fisheries and Conservation Department.

The city's government is tracing the source of the carcass and it is not known whether the chicken was from a local farm or imported.

All 30 chicken farms in Hong Kong were inspected on Tuesday. No abnormality was detected.

Local farms are prohibited from sending chickens to the market for 21 days. The Agriculture, Fisheries and Conservation Department will step up farm inspections and collect more samples for tests.

Meanwhile, imports of live poultry including day-old chicks would be suspended for 21 days.

"It is unfortunate that an avian influenza case is detected before the winter solstice, necessitating a halt to the supply of live chickens," Chow said.

"I understand that it will cause inconvenience to the public, and the poultry trade will also encounter losses. However, to safeguard public health, we need to adopt decisive and effective measures to prevent and control the spread of the virus."