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HONG KONG -- Hong Kong health authorities said Tuesday they would slaughter more than 80,000 chickens after three dead birds tested positive for the H5 avian flu virus. Officials are trying to determine how the infections occurred despite the city's efforts to prevent bird flu, in the first group of new cases on a farm here in more than five years.

The news could hit public sentiment as the financial center struggles with the impact of the global economic slowdown. Hong Kong, a special administrative region of China, fell into recession in the third quarter, and the city's leader warned this week that the territory faces a difficult 2009.

"This incident shows us that it is no guarantee that we would not have an avian influenza outbreak in our farms," York Chow, Hong Kong's secretary for food and health, said Tuesday. Mr. Chow said the chickens were found Monday on a farm with 60,000 birds that has since been designated an infected zone. Officials concluded from the three tested birds that the roughly 60 that died there carried the bird flu. He added that Hong Kong would suspend poultry imports for 21 days and begin slaughtering more than 80,000 birds.

Officials said they hadn't determined if the virus they found was the H5N1 strain of bird flu, which has proven deadly for humans. Nor have they determined the source of the infection.

Yi Guan, a microbiologist and avian flu expert at Hong Kong University, said Hong Kong has some of the highest safety standards in the region but warned of "leaking holes" in the system. "We have a high-tech biosecurity system," Mr. Guan said. "But how the farmer runs the farm is another story."

Mr. Guan said it would take scientists "a couple of days" to determine the particular strain of the virus.

In June, a routine inspection of an outdoor food market turned up five cases of avian influenza, spurring officials to slaughter chickens and suspend supplies of live chickens from local and mainland Chinese farms for three weeks. An investigation was launched to determine the source of that infection, but has been inconclusive. A government spokeswoman said Tuesday that determining the source "posed some difficulties."

So far, bird flu's impact has been muted by its inability to easily pass from human to human. Since 2003, it has infected 389 people in 15 countries, including China, Indonesia and Vietnam, according to the World Health Organization. Of those confirmed cases, 63% have proved fatal. Scientists worry that the flu could mutate into a tougher and more contagious form.

Hong Kong has seen occasional bird-flu incidents but no major outbreaks since 1997, when the virus killed six people and led to a slaughter of the territory's 1.5 million birds. There are currently about 600,000 poultry birds in Hong Kong, according to Hong Kong's government, which is discouraging vendors from selling live chickens.

Mr. Guan, said now would be a good time to review current safety standards. "After this event, we will go and check to see whether the system is good or not, or if it can be improved," he said.