A Cambodian woman who died in Vietnam at the weekend was killed by bird flu, health officials have said.
Human bird flu resurfaced in Vietnam at the end of December
The woman, Tit Sakhan, is the first confirmed, non-Vietnamese death in the latest outbreak.
Although she died in Vietnam, she is believed to have caught bird flu in Cambodia, where her brother died of similar symptoms last month.
Thirteen people are now confirmed to have died of the disease since its resurgence on 30 December.
Later this month, Vietnam will host an international conference to discuss ways of combating the spread of the disease.
Tit Sakhan first became ill at her home in Cambodia's Kampot province, but like many Cambodians who can afford the cost, she travelled to Vietnam for medical treatment.
Nine days after the first symptoms, she died in a hospital in Kien Giang, near the Vietnamese-Cambodian border.
The Pasteur Institute in Ho Chi Minh City has now confirmed that bird flu was the cause of death.
The confirmation will concern Cambodian officials, since Tit Sakhan's younger brother died nine days earlier of similar symptoms. Their father said two other members of the family were also sick.
A team for the World Health Organisation has carried out an investigation at the family's home, but is yet to reveal its their findings.
The resurgence of bird flu has renewed scientists' fears that the virus could mutate into a form that is easily spread from human to human.
H5N1 BIRD FLU VIRUS
Principally an avian disease, first seen in humans in Hong Kong, 1997
Almost all human cases thought to be contracted from birds
Isolated cases of human-to-human transmission in Hong Kong and Vietnam, but none confirmed
Officials are investigating the Cambodian case and also another case in Vietnam, where a 13-year-old girl and her mother both died of the virus, for possible human-to-human transmission.
The strain of virus causing the deaths - H5N1 - is suspected of having jumped from human to human before, most recently in September last year, when a mother in Thailand probably became infected from her daughter.
But this was an isolated case. What experts are especially worried about is that the virus could combine with a human flu virus, if someone - or possibly an animal - became simultaneously infected with both.
If these viruses then exchanged genes, a new, highly infective virus could be created and be easily passed from person to person.
Officials in Vietnam are taking tough measures to slow the spread of infection among poultry.
All the ducks in Ho Chi Minh City are due to be slaughtered, according to Phan Xuan Thao, deputy director of the municipal animal health bureau.
Ducks can carry the deadly H5N1 virus without showing symptoms.
About 55,000 ducks that tested negative for the disease will be slaughtered and frozen for sale, while another 125,000 will be destroyed, he told the Associated Press news agency.
Vietnam has already killed or slaughtered about a million birds nationwide since the start of 2005.
"The next 10 days will be a critical time, as the increase in poultry transportation and cool temperature favours the spread of the virus," said Bui Quang Anh, director the Animal Health Department in Hanoi, referring to the preparations for the approaching lunar New Year festivities.
In a joint statement, the United Nations Food and Agriculture Organisation (FAO) and the World Organisation for Animal Health (OIE) said the disease would be present in affected nations for several years.
"The international community has to realise that some poor countries in Asia living with the bird flu virus must receive more support," they said.
"The chances for spillover from ducks inhabiting the vast wetlands to poultry production units in villages or on commercial farms need to be significantly decreased," the statement said.