The World Health Organization says Asia is facing one of the largest bird flu outbreaks ever recorded, which in turn poses a threat to humans.

The United Nations health agency says the size of the outbreak of avian influenza in Asia is unprecedented. The H5N1 virus, which is linked to five human deaths in Vietnam, has been found in poultry in South Korea, Japan, Taiwan and Vietnam.

Some experts say efforts to contain the disease in Vietnam are complicated by the Lunar New Year holiday, also known as Tet. Government offices are closed for the next few days and it is not clear if state-organized culling of poultry will continue through the holiday.

Bob Dietz, a WHO spokesman in Hanoi says the outbreak in chickens is not yet under control and some human cases may be going unnoticed.

"[It's] hard to tell from the limited window we have, the disease surveillance system, whether we're seeing a full clear picture," he said.

Vietnam doctors were testing 17 people for all known types of avian influenza.

In the past two months, the virus has spread quickly to bird farms around the region. On Wednesday, it was found in a wild bird in Hong Kong.

Lai Ching-wai is with Hong Kong's Agriculture, Fisheries and Conservation Department. "We have collected a carcass of a peregrine falcon on Monday, and through the test in the laboratory we discovered the bird had been infected with the highly pathogenic avian influenza H5N1," said Lai Ching-wai.

Mr. Lai says poultry farms in the territory are being monitored, but so far Hong Kong's chickens have been spared.

The bird flu first crossed to humans in Hong Kong in 1997. Six of the 18 people infected then died.

On Wednesday, Thailand's government said it is slaughtering chickens infected with cholera. The government says it has found no sign of bird flu in the country.

There are also are unconfirmed reports that chickens falling ill in Cambodia and Laos.

China says it is free of the bird flu, although it is monitoring poultry farms. The government also is watching for cases of Severe Acute Respiratory Syndrome, or SARS. The disease infected 8,000 people in just eight months after it emerged in China in late 2002.

So far this year, three people in southern China were confirmed to have caught SARS. The third patient was released from the hospital on Wednesday, the other two recovered several days ago.