In neighbouring Thailand, health workers have been put on alert and ordered to conduct stringent surveillance measures but it is Vietnam that has emerged as the epicentre for the deadly H5N1 virus.
A 48-year-old man, his younger brother and a 62-year-old man were hospitalised on Saturday with acute pneumonia, which doctors in Hanoi suspected could be caused by bird flu, the Quan Doi Nhan Dan daily reported on Monday.
The 48-year-old man died and the other two patients were in critical condition. If the cause of the man's death is confirmed as bird flu it would be the first fatality in the country's north from the virus that has killed 37 people in Asia - 25 in Vietnam and 12 in Thailand.
A fourth patient, a 14-year-old boy, was taken to a Ho Chi Minh hospital with high fever and coughing on Sunday from the southern province of Tra Vinh, according to the Labourer newspaper.
Contact with poultry
Government officials were not immediately available for comment.
The human victims are believed to have caught the virus from contact with sick poultry but the World Health Organisation has warned that the virus could mutate and spread out of control among people, killing millions.
The agriculture ministry said bird flu had spread to ducks and chickens in the Ninh Thuan and Quang Nam provinces in Vietnam's central region. The virus has emerged in all of the country's three regions.
The poultry virus has killed 37
people across Asia
Officials said some 234,000 chickens, ducks and birds have died or been slaughtered because of the virus since the beginning of the year.
Vietnamese researchers will soon start testing a vaccine on monkeys, chickens and mice with the hope of using it on humans and poultry, the Labourer newspaper said.
It quoted Pham Ngoc Dinh, deputy head of the Institute of Hygiene and Epidemiology, as saying the vaccine has been under research since April.
Last year, bird flu destroyed 17% of Vietnam's poultry stock of 250 million.