It was still unclear whether the man contracted the virus from the patient or infected poultry, the doctor said, adding further investigations would be carried out.

"He is the nurse who took care of a patient confirmed positive to H5N1," a doctor from Hanoi´s Institute of Tropical Diseases told AFP.

The patient, 21, from northern Thai Binh province, has been in critical condition for the last 10 days.

A 69-year-old man from the same province of Thai Binh, 100 kilometres (60 miles) from Hanoi, died of bird flu on February 23.

Four people are now hospitalized with the disease in Hanoi, three of whom are said to be in stable condition.

Some experts believe the disease could trigger a devastating pandemic if the H5N1 virus mutates to a form easily transmissible among humans.

Earlier this year, US researchers said they had the first scientific evidence to confirm worries the avian virus could be transmitted from one human to another.

According to researchers from the Thai public health ministry and the US Centers for Disease Control and Prevention, an 11-year-old child who died in a Thai village last year after contracting the flu from a chicken probably transmitted the virus to her mother and her aunt.

The scientists established that the 26-year-old mother and 32-year-old aunt were exposed to the illness while caring for the girl. But the two women had not come into contact with infected birds.

Some 34 people have died in Vietnam since late 2003 in several outbreaks of the disease, including one Cambodian national. Another 12 have died in Thailand.

Thirty-five of Vietnam´s 64 provinces and cities have been hit by bird flu this year and more than 1.5 million poultry have been destroyed in a bid to control the disease.

According to a daily report by the Ministry of Agriculture, the bird flu situation in Vietnam has abated with 17 out of 35 hit provinces reporting no new outbreaks for at least 21 days.

Last week, the World Health Organisation in Hanoi said the current outbreak was probably waning, adding the latest reports of human cases in the country were not necessarily alarming.

In February, WHO regional director Shigeru Omi warned at an international conference on bird flu in Ho Chi Minh City last month of "the gravest possible danger of a pandemic."

During the meeting, Vietnam vowed to implement a nationwide overhaul of the poultry industry as part of efforts to stamp out bird flu.