UN agencies called on Tuesday for Vietnam to be vigilant in fighting bird flu after three new human cases were discovered this year, including one woman who died.
The World Health Organisation (WHO) and the UN's Food and Agriculture Organisation (FAO) said Vietnam, which has seen more bird flu deaths than any country except Indonesia, had to monitor the situation closely.
"While avian influenza poses a threat to humans and poultry throughout the year, experience in Vietnam has shown that the threat is highest at this time of year," Jean Marc Olive, a Vietnam-based WHO official, said in a joint statement.
"There is a real possibility that the virus will mutate and become easily transmissible between humans. If this occurs we could see the start of a new influenza pandemic."
The WHO and FAO said that, while it is still relatively difficult for humans to be infected by the H5N1 virus, about 50 percent of those infected die.
"The avian influenza virus is also not currently able to spread easily between humans, but influenza viruses are known for their ability to change quickly and can become more adapted to humans with dire consequences," the statement said.
On Saturday a woman died from the H5N1 strain of the virus in Vietnam's first avian influenza fatality of the year.
An eight-year-old girl, infected in January, has since recovered while a third victim is still in a Hanoi hospital.
Ten Vietnamese provinces are currently experiencing bird flu outbreaks.
Vietnam managed to contain the virus in 2006, but in 2007 recorded new cases of human infection and deaths. Last year there were five deaths from the H5N1 virus, all in the first quarter of the year.