Vietnam has confirmed a lethal H5N1 bird flu outbreak among its domestic poultry in the south of the country.
Bird flu has killed 42 people in Vietnam since 2003 (Source: WHO)
Some 5,500 ducks and 500 chickens have died in the last two weeks in two provinces on the Mekong Delta, health ministry officials said.
The government said there was "an extremely high risk" of it spreading widely in the delta region.
Vietnam has been free of human cases of bird flu for a year, following a large culling and vaccination drive.
The last known major outbreak among birds was in December 2005.
In the latest cases, all the poultry found dead were hatched illegally in the provinces of Ca Mau and Bac Lieu, health officials said.
"The risk of bird flu widely spreading in the Mekong delta is extremely high because farmers have thrown dead poultry into water channels for a long time," the Animal Health Department said in a report.
Bird flu first arrived in the Mekong delta in late 2003 and has since killed 42 people in the country.
As of 29 November 2006, the World Health Organization confirmed 258 cases of H5N1 in humans worldwide, leading to 154 deaths.
Vietnamese officials have been calling on people to remain vigilant as the Northern Hemisphere winter sets in.
In previous years, most human cases have been recorded between December and May, and the H5N1 virus survives longer in cooler conditions.