Vietnam on Tuesday confirmed two new outbreaks of bird flu in domestic poultry, the first in more than a year in the country once hardest-hit by the deadly H5N1 strain.

Thousands of chickens and ducks will be slaughtered in two Mekong Delta provinces to prevent the spread of the disease, which has killed 158 people and hundreds of millions of chickens worldwide.

The H5N1 strain of bird flu is not easily contagious among humans, but scientists fear if it is left unchecked it could mutate into a new human influenza pandemic strain, threatening the lives of millions of people who would have no natural immunity.

Earlier this year, Vietnam - which has seen 42 people die from bird flu since 2003 - declared it had brought bird flu under control through an aggressive programme of vaccinations.

No deaths have been reported in either poultry or humans since November 2005.

However, the mass deaths of chickens and ducks last week in southern Ca Mau and Bac Lieu provinces prompted tests that were positive for the H5N1 strain of avian influenza, according to Hoang Van Nam, director of the Epidemic Unit under Vietnam´s Department for Animal Health.

"The farms where poultry was found dead have been ordered to cull all the live poultry and to disinfect all the areas surrounding," Nam said.

The dead chickens and ducks - most just one month old had apparently been hatched illegally and had not been vaccinated, he added.

"The possibility that the outbreaks will spread to other farms is very high, especially to the farms where the poultry has not been vaccinated," Nam said. "The farmers didn´t report to local animal officials about the death of the poultry at first hand until the number of dead poultry is so high."

Vietnam was the epicentre of the H5N1 crisis when the virus re-emerged in 2003, with millions of poultry dead and more human victims - eventually 42 - than anywhere in the world at one point.

After the vaccination programme controlled the virus in poultry, Indonesia overtook Vietnam as the hardest-hit by the disease, with 57 people dead as of this month.

Vietnam has vaccinated 83 million chickens and 43 million ducks in the second vaccination round of this year.

Health experts hope to control the virus in poultry because contact with diseased birds is the main way humans can catch the deadly disease - which has killed about 60 per cent of people known to have contracted the virus.