Six people from the US Centres for Disease Control and Prevention, including three epidemiologists, flew into Hanoi late on Monday to help the World Health Organisation and Vietnamese authorities.
"These are people who are specifically trained to come into situations like this running," said Bob Dietz, the UN health agency's spokesman.
The bird flu outbreak, which was first detected in early January, has cast a shadow over Tet, the Vietnamese festival marking Thursday's start to the Lunar New Year.
Dietz, however, has said the WHO has been assured by the government it will not take its eyes off the crisis during the week-long holiday when the entire country virtually grinds to a halt.
Stream of patients
The WHO spokesman said a constant stream of patients continued to be admitted to two hospitals in Hanoi with respiratory symptoms.
"This doesn't necessarily mean we are seeing more H5N1 cases. They are being tested for the disease and this is a lengthy process," he told AFP.
"But this is not a valid indication of what is going on in the country. It is a very narrow window. The situation could be far worse elsewhere or it could be better."
The five deaths in Vietnam confirmed to have been from the H5N1 virus have occurred in the north, baffling experts as to why no cases have been reported in the south where the infection rate among poultry is high.
Vietnamese health officials have attributed at least half a
"These (US experts) are people who are specifically trained to come into situations like this running"
UN health agency spokesman
dozen other deaths at the two Hanoi hospitals to the disease. Nine patients are currently being treated at the Central Paediatric Hospital with suspected bird flu.
The latest victim was an eight-year-old girl from the northern
province of Ha Tay, who died on Saturday.
No confirmed or suspected deaths have been reported in South Korea, Japan and Taiwan, which are all coping with their own bird flu outbreaks.
The Thai government played down claims on Tuesday that a chicken butcher suffering from a respiratory ailment was infected with bird flu, saying tests were still continuing.
It has insisted that its valuable poultry industry has escaped the virus, but instead is battling an epidemic of fowl cholera and bronchitis that has resulted in the death or slaughter of at least 850,000 chickens.
Ban on poultry imports
Laos, meanwhile, became the latest country to prohibit imports of poultry and poultry products from Vietnam, but it also extended the ban to Thailand.
The WHO believes the five victims in Vietnam contracted the H5N1 virus through contact with droppings from sick birds. It says there is still no evidence of human-to-human transmission.
But the UN agency has warned that if the virus mutates to travel through human contact, it could spread rapidly, triggering a far greater health crisis than last year's SARS outbreak, which killed almost 800 people worldwide.