Image caption Hong Kong is quick to take action against infectious diseases

Hong Kong is culling 17,000 chickens after three birds were confirmed to have died from the deadly H5N1 bird flu strain in the past week.

The government has banned imports and the sale of live chickens for three weeks after an infected chicken carcass was found at a wholesale market on Tuesday.

It has also raised the city's flu alert system to "serious".

Two wild birds were also found to have died of the the virus.

The government said it was tracing the source of the chicken carcass, but it was not clear whether the chicken came from a local farm or was imported.

"I understand that it will cause inconvenience to the public, and the poultry trade will also encounter losses," said Hong Kong's secretary for food and health, York Chow.

"However, to safeguard public health, we need to adopt decisive and effective measures to prevent and control the spread of the virus."

On Tuesday, a dead Oriental magpie found at a secondary school tested positive for bird flu.

Another secondary school closed for a day for disinfection last Friday after a dead black-headed gull was found with the virus.

Hong Kong is quick to take action against infectious diseases after an outbreak of the deadly respiratory disease SARS in 2003 killed 300 people in the city and a further 500 worldwide.

In 2009, 300 people were placed under quarantine at a Hong Kong hotel after a guest contracted swine flu.