Health workers began culling thousands of chickens at a market yesterday, a day after authorities raised the bird flu alert level to "serious" following an outbreak at a farm.

The outbreak near the border with the mainland was the region's first in five years and came despite mass vaccination of birds, prompting concerns the virus might have mutated.

Ho Pak-leung, a microbiologist at the University of Hong Kong, said: "The vaccine's effectiveness today will not be the same as it was five years ago."

A spokeswoman for the Hong Kong government said: "We aim to complete the cull as soon as possible," without giving a specific timeframe.

Health authorities in the region raised the bird flu alert level to "serious" on Tuesday after the H5 virus killed dozens of chickens at a farm.

Scientists are now trying to determine the identity of the virus. One said it is likely to be the highly pathogenic H5N1 strain, which turns up regularly in flocks in Asia.

Although H5N1 is mainly a disease among birds, it may mutate into a form that spreads easily among people.

Mao Qun'an, spokesman of the Ministry of Health, told a press conference yesterday that there is a possibility of people becoming infected.

"One of the best ways to reduce the risk is to avoid direct contact with birds, especially dead ones," he said.

On Tuesday, authorities in Hong Kong issued a 21-day suspension on the import of live poultry from Guangdong in a bid to contain the outbreak.

The ban does not include frozen poultry, officials said.

The flu outbreak is the second in Hong Kong this year. The first was on June 6, when chicken excrement collected from poultry markets tested positive for the H5N1 strain, leading to a cull of all poultry.

Editor: Yan