Two United Nations bodies warned that renewed vigilance was necessary to prevent the further spread of deadly bid flu in Vietnam, which has reported three new cases in as many months, with one person dying.

The H5N1 strain has infected poultry in 10 Vietnamese provinces, and at the weekend a 23-year-old woman died of the disease in the northern province of Quang Ninh.

"The animal and public health sectors need to work closely together to minimise the ever-present threats of avian influenza and other animal-based diseases passing to humans," said Andrew Speedy, Vietnam representative of the Food and Agriculture Organisation (FAO).

Jean-Marc Olive, representative of the World Health Organisation (WHO), said there was a "real possibility" of human-to-human transmission.

"If this occurs we could see the start of a new influenza pandemic," he said in a joint WHO-FAO statement.

In 2008, five people died of avian influenza in Vietnam.

Avian influenza has affected millions of birds around the world, particularly in East Asia, and can be caught by humans who come into contact with infected birds -- a big danger in those societies where people keep poultry in the yards of their homes or live close to food markets.