HANOI - A 22-year-old pregnant woman has died of bird flu in Vietnam, medical sources said Tuesday, bringing the death toll in the country to three in less than two months.

The woman, who lived in Ha Tay province outside the capital Hanoi, died Saturday after being admitted a week before for treatment, said a source at a Hanoi hospital who asked not to be named.

"Tests were positive for the H5N1 virus," the source said.

Her death brings to 45 the number of people who have died of bird flu in Vietnam. Two people -- a 20-year-old man and a 28-year-old woman -- died in June of avian influenza, the first fatalities announced since November 2005.

Since May, six human cases have been reported here, three of them fatal.

Vietnam, once the nation worst hit by bird flu, contained earlier outbreaks through mass vaccination campaigns, the culling of millions of poultry and public education campaigns.

But the virus has come back strongly this year, hitting scores of poultry farms in an unusual summer-time outbreak.

Outbreaks have been reported since early May across 18 of Vietnam's 64 provinces and municipalities, mostly among unvaccinated ducks and other waterfowl.

As of last week, the World Health Organisation had recorded 319 cases of bird flu in humans worldwide, 192 of which were fatal.

Experts fear the death toll could rise sharply if the virus were to mutate and become easily transmissible between humans.

- AFP /ls