A girl has become the fifth person to die from bird flu in Vietnam, the World Health Organization has confirmed.
Two million chickens have died or been slaughtered in Vietnam
The eight-year-old girl, from the northern province of Ha Tay, died at the weekend, officials said.
On Saturday, WHO officials said a five-year-old boy in the north of the country had died from the virus.
Millions of chickens in Japan, South Korea and Taiwan have been infected with bird flu, but only Vietnam has yet confirmed cases of the human disease.
WHO officials say there is no evidence that the disease is spreading directly between humans.
"The source of infection still seems to be coming from poultry," group spokesman Bob Dietz told the French AFP news agency.
"There still does not seem to be human-to-human transmission but we are monitoring the situation very closely."
The bodies of another eight Vietnamese people who have died of influenza are being tested to see if the strain is the H5N1 bird flu.
BBC correspondent Kylie Morris says the girl's death is alarming news for authorities trying to control the epidemic in Vietnam.
On Saturday, hospital officials in Vietnam's southern Kien Giang province said they had two new patients with bird flu symptoms - high fevers, coughs, low blood pressure and low levels of blood cells.
It is the first time that suspected human cases have been reported in the south. All of Vietnam's confirmed cases - as well as the eight deaths under investigation - were from the north.
"We believe we are seeing more cases of respiratory illness," Mr Dietz said.
But he stressed that it was "too early to say whether or not this indicates an increase in H5N1 cases".
The virus has resulted in the death or slaughter of two million chicken in Vietnam, and the authorities have ordered the cull of all chickens in the 12 worst-hit provinces.
The sale of poultry was banned in Ho Chi Minh City - Vietnam's largest city.
On Saturday, Taiwan ordered 35,000 more chickens to be slaughtered after the less virulent H5N2 strain of the virus was found at a second farm in one week. Some 20,000 birds were culled on Friday.