A 13-year-old boy has become Vietnam's sixth victim of avian flu, the World Health Organization (WHO) has said.
The disease is linked to close contact with live chickens
Five Vietnamese children and one adult have now died from the virus, which has also crossed from birds to humans in Thailand.
The WHO has warned that the outbreak could mutate and become more dangerous.
Singapore, Malaysia, Taiwan and South Korea have joined a growing list of countries to ban imports of poultry from Thailand, Asia's main exporter.
Thailand's main poultry buyers Japan and the European Union had already banned Thai chicken, along with the Philippines, Hong Kong and Bangladesh.
AVIAN FLU ALERT
First jumped "species barrier" from bird to human in 1997
In humans, similar symptoms include fever, sore throat, and cough
Types known to infect humans are influenza A subtypes H5N1 and H9N2
Avian flu has also affected chickens in Cambodia, Taiwan, Japan, and South Korea - but is not known to have jumped from birds to humans in these countries.
The latest victim of the Vietnamese outbreak died on Thursday in Ho Chi Minh City - the first confirmation of the virus in the south of the country.
The WHO also announced that an eight-year-old girl in the city had tested positive for the virus and was in a critical condition.
Friday saw Thailand confirm its first two cases of avian flu in humans.
Bangkok said two boys had contracted the virus after touching carcasses of infected poultry - one boy in Suphanburi and one in Kanchanaburi province.
AVIAN FLU TIMELINE
Nov 2003 - Thailand reports what it calls chicken cholera
15 Dec - S Korea confirms avian flu outbreak
9 Jan 2004 - UN sends help to Vietnam after avian flu outbreak
11 Jan - First of five Vietnamese deaths confirmed as avian flu
13 Jan - Japan confirms avian flu outbreak
15 Jan - Taiwan announces different strain of avian flu
21 Jan - Laos reports suspected chicken cholera
23 Jan - Thailand confirms first human cases of avian flu
23 Jan - Cambodia detects first case in chickens
Medical authorities also announced the death of a 56-year-old Thai who raised fighting cocks at his home near Bangkok. He was one of six people in the country being tested for avian flu.
The authorities say anyone suffering from fever and bronchitis after having been in contact with poultry should seek urgent medical attention.
Thai Prime Minister Thaksin Shinawatra has denied accusations that his government tried to cover up the outbreak of avian flu.
"There has been a lot of talk that the government has been trying to cover this up," Mr Thaksin said in a weekly radio address on Saturday.
"That we didn't say anything doesn't mean we weren't working. We've been working very hard."