The avian flu has killed millions of chickens in Vietnam, South Korea and Japan in the past month, triggering a health scare across Asia where many countries have banned poultry imports from affected areas.

So far the only human infections from the H51N virus have been in Vietnam where authorities have recorded 18 suspected cases and 13 deaths.

In the latest cases, an adult died on Wednesday and three people showing symptoms were isolated at a Hanoi hospital.

"Eighteen people have been found to have contracted influenza A, of whom 13 died, including 11 kids and two adults," the official Vietnam News Agency said.

The infections are believed to have come from contact with droppings from sick birds, and both World Health Organization and Vietnamese experts have stressed there is no evidence so far to show the virus can spread from person to person.

However, the WHO, which has confirmed three deaths from the H51N virus and is doing tests on some other cases, has warned of dire consequences if the virus mutates.

Professor Hoang Thuy Long, director of the Institute of Hygiene and Epidemiology at the Vietnamese health ministry, said the Vietnamese authorities were taking the warning very seriously.

"If the virus continues to evolve it could become contagious between humans. We would then see a very large flu epidemic indeed," he told AFP.

Analysis so far suggests all the genes from the H5N1 virus come from animals.

"If a gene was of human origin, there would be a far greater danger of human to human transmission. But it appears this is not the case, which is positive," said Pascale Brudon, WHO representative in Hanoi.

Another positive sign is that so far no medical staff treating bird flu patients have become infected, unlike with last year's outbreak of Severe Acute Respiratory Syndrome when many doctors and nurses became sick.

However experts have by no means excluded person-to-person transmission, especially as the virus has struck several members of a single family.

The first adult to die last Saturday was the mother of a young girl who died several days earlier. Hospital sources also said the three latest cases were family members of the man who died on Wednesday.

In both cases experts have so far been unable to confirm whether the source of the illness was animal or human.

Smaller outbreaks in South Korea and Japan, which have yet to be eradicated, have created a new anxiety in Asia which is also coping with the re-emergence of SARS in southern China.

Hong Kong, where bird flu killed six people in 1997, has banned all live poultry from affected areas. Cambodia, Thailand, Taiwan and China have also announced restrictions and bans.

In South Korea the highly contagious disease reappeared after hitting 15 areas nationwide last month despite a cull of 1.8 million chickens and ducks.

Japan has slaughtered over 8,000 chickens but has appealed for calm, insisting the virus is contained to a single farm.

First signs in Vietnam of the H5N1 avian influenza virus emerged nearly three weeks ago in the southern Mekong Delta provinces of Long An and Tien Giang around Ho Chi Minh City.

However, it subsequently spread to other delta provinces and outbreaks were reported in the north.

Vietnam's agriculture ministry has ordered the destruction of all chickens suspected of contracting the virus, although containment has been complicated by next week's Lunar New Year festival, when chickens are widely eaten.

Traders at poultry markets in Ho Chi Minh City said business had slumped.

The authorities have released 6 million dollars to fight the epidemic.

TERRA.WIRE