Vietnam is on the brink of another bird flu epidemic in poultry, the agriculture minister told provincial authorities on Wednesday after the H5N1 virus has spread rapidly over the past month.

The virus has infected ducks and chickens in 11 provinces and Can Tho city in May at the beginning of summer. The development is unusual because experts say the virus normally appears to thrive best in cool temperatures and weakens in warmer weather.

Last week, the Southeast Asian country reported its first human case of H5N1 bird flu virus infection in a year and a half, a 30-year-old man in a province neighbouring Hanoi.

"The recent outbreaks were found mainly in waterflowl flocks that have not been vaccinated," Agriculture Minister Cao Duc Phat said in what was described as an urgent telegraph to People's Committees in all 64 provinces and cities.

"Now the development of the epidemic is very complicated. The risk of the epidemic's further development and spreading on a large scale is very high," Phat's message said.

It called for the vaccination of "100 percent of ducks".

Bird flu has killed 42 people in Vietnam since it re-surfaced in Asia in late 2003.

The communist-run country took drastic steps in 2004 and 2005 to control the nationwide spread of the virus, including mass vaccination of poultry and banning the sale of birds in markets in major cities.

But it was flared up repeatedly in rural areas, including a number of southern provinces early this year.

Tests confirmed the H5 component of the virus in 14-day-old ducklings on a farm outside Haiphong city last week, the second outbreak in the area, the ministry's Animal Health Department said in a report seen on Wednesday.

It killed 2,120 fowl and animal health workers slaughtered the remaining 1,200 ducklings.

Other outbreaks were reported on Wednesday in the northern provinces of Ha Nam and Vinh Phuc, home area of the hospitalised man, who was apparently infected while slaughtering chickens.

The number of birds killed by the virus and slaughtered this month is more than 50,000 nationwide.

The full economic impact might not be known for some months until the government compensates farmers for slaughtering their poultry, a World Bank economist said.

"The consequences will be much stronger in the winter," said Martin Rama, acting director of the World Bank in Vietnam.