The United Nations' health agency said Sunday that the bird flu, which has killed millions of chickens across Asia, might have spread among members of a family in Vietnam.

Two sisters died a week ago; they fell ill after their brother died of an unidentified disease. His wife also contracted the bird flu, but has recovered.

WHO epidemiologists say they found no conclusive evidence linking the sisters to infected poultry. Laboratory test carried out in Hong Kong confirmed they had the H5N1 virus.

Experts say the sisters, aged 23 and 30, may have caught the disease from their brother, who was cremated after he died, before any tests were done on him.

The WHO spokesman in Vietnam, Bob Dietz, says experts are considering the possibility of human-to-human transmission. "This is not confirmed human-to-human transmission, only that, at this point, we cannot dismiss it as not a possibility," he said."

Scientists have long warned the H5N1 virus poses a potential threat to global health if it becomes able to spread as quickly among people as it does among chickens.

Bird flu has spread to at least 10 countries in Asia. The H5N1 virus is responsible for 10 human deaths in the past several weeks. Scientists have linked almost all human cases to contact with sick birds.

The family in Vietnam are not the first that epidemiologists have suspected of human-to-human transmission of the disease.

In 1997, when the H5N1 virus first crossed from chickens to humans in Hong Kong, a doctor fell ill with the disease, which he may have contracted from a patient.

"The doctor became ill and was shown to be H5N1-positive," said the WHO spokesman. "The most plausible explanation was that he contracted it from the patient."

Six of the 18 human cases in Hong Kong in 1997 died. Except for the doctor, all had been exposed to poultry.