HONG KONG (AFP) - Hong Kong said Wednesday it would slaughter all live poultry in markets and shops around the city following a fresh outbreak of bird flu.

"We have announced that all wet market stores and fresh food stores selling live poultry are now infected areas," said Cheung Siu-hing, director of the agriculture, fisheries and conservation department.

Cheung said the authorities had "launched a cull on all the chickens in those stores."

The cull comes after the deadly H5N1 virus was found in chickens in three more markets across the city, Cheung told reporters, following an outbreak announced Saturday.

She said chickens at 470 stores across the city would be killed.

Local broadcaster RTHK said about 3,500 birds will be slaughtered. However, poultry in local farms would not be affected, RTHK said.

All samples taken from the 50 local chicken farms in the city tested negative for bird flu, said Thomas Sit, assistant director at the agriculture, fisheries and conservation department.

The move follows an outbreak of the virus on Saturday that led Hong Kong authorities to suspend live poultry imports from mainland China. It was the first outbreak among chickens in Hong Kong wet markets in five years, reports said.

Thomas Tsang, controller of the city's Centre for Health Protection, said Wednesday there had not been any human infections, but authorities would step up monitoring.

"I believe we have to continue to be on high alert in the near future," he said.

There have been several outbreaks of bird flu in the territory this year, but only among migratory birds, which have been blamed for the spread of the disease worldwide.

Secretary for Food and Health York Chow said authorities were working to trace the source of the bird flu and they were suspending live chicken imports from the mainland for 21 days.

Hong Kong was the scene of the world's first reported major bird flu outbreak among humans in 1997, when six people died.

The H5N1 strain has killed more than 240 people and ravaged poultry flocks worldwide since 2003, according to the World Health Organisation.

Scientists fear the virus could mutate into a form that is much more easily transmissible between humans, triggering a global pandemic.