"We are afraid that this might be a watershed moment," WHO spokesman Peter Cordingley said in Manila.

Tet, during which much chicken is eaten, is being celebrated next week, but bird flu has already spread to half the country's 64 provinces since erupting again in December.

"People are going to be on the move and there's going to be a lot of chickens that will be moved around," Cordingley said.

Almost all human victims of the disease, which kills about 80 percent of them, got it from direct contact with infected fowl and Cordingly said governments had to do more to teach people not to pluck and eat fowl already dead or dying.

"We understand why this is being done, because for many families the chickens in their backyards are their main source of food," he said. "We are advising governments in the region that the message must go out that people have to be more careful."

Vietnam's third epidemic of the H5N1 virus since the strain arrived in Asia at the end of 2003, probably brought by migrating wildfowl, began in the Mekong Delta.

It has spread relentlessly despite increasingly urgent measures to halt a disease which has now killed 45 people -- 32 Vietnamese, 12 Thais and one Cambodian.

"ALL KINDS" OF HELP NEEDED

So the government has turned to the WHO and the UN's Food and Agriculture Organization.

"The government has asked for help and in a recent meeting with the WHO and FAO, the agriculture minister also asked for all kinds of bird flu fighting measures," Bui Quang Anh, head of the ministry's Animal Health Department, told Reuters.

Vietnam, which will host a conference on bird flu in Ho Chi Minh City on Feb. 23, has asked for equipment, help in surveillance and other forms of expertise, Anh said.

The government of the city, home to 10 million people, has set a Sunday deadline for the slaughter of its more than 200,000 ducks, which can carry the H5N1 virus without showing symptoms, and pigeons reared for food.

The city has not yet reported a human case of bird flu, which is fatal in about 80 percent of cases, but the Mekong Delta is nearby. Checkpoints have been set up around the city to prevent farmers bringing in live fowl.

Experts say the longer outbreaks last and the wider they spread, the closer a human pandemic comes because it gives the H5N1 virus more chances to enter an animal, most probably a pig, which can also carry human flu viruses.

That would allow it to mutate into a form against which humans have no immunity and to sweep through the world's population, they say.

The WHO already fears the virus may have reappeared in Cambodia, Laos and Myanmar, all poor countries where surveillance systems are weak at best.

It has appeared again in Thailand, one of the countries hardest hit last year, with outbreaks in 7 of its 76 provinces, up from five late last week.

But officials in Phnom Pehn said initial tests around the home of a Cambodian woman who became the country's first victim had proved negative.

But they said more tests were being conducted on poultry in the area of Kampot province along the porous border with Vietnam. A team of WHO experts has rushed there and they await definitive word from laboratories on whether bird flu has reached it.