Fears over Asia's bird flu (search) outbreak spiraled Wednesday as worst-hit Vietnam admitted that nearly 900,000 chickens possibly exposed to the deadly virus had been sold to the public.

The disease may have spread to nearby Thailand -- a major chicken exporter that has repeatedly insisted it is free of bird flu -- where three people were being tested for the avian influenza. A mass slaughter of fowl was ordered by the government.

The bird flu ravaging poultry farms in Asia has killed five people, all in Vietnam, and millions of chickens. International health experts are trying to find a vaccine for humans.

Struggling to contain the epidemic, jittery governments have banned poultry imports from countries affected by the disease. Mainland China, which has not reported any cases, said it would step up vigilance at its border with Vietnam.

The World Health Organization (search) expressed "mounting concern" over the five human deaths and said it was working on a new vaccine to protect people from the avian flu.

But making sure the vaccine is safe for the public could be a lengthy process, WHO spokesman Bob Dietz said in Hanoi Wednesday. "It could be several months to several years" before it's ready, he said.

News that potentially sick birds were sold to the public in Vietnam before a mass destruction of fowl was ordered prompted new health worries there.

"There've been nearly 900,0000 chickens that farmers have sold to the market from the beginning of January, mostly from Long An and Tien Giang," said Nguyen Van Thong, deputy director of the veterinary department under the Ministry of Agriculture and Rural Development, referring to the two hardest-hit provinces. The chickens were still alive when sold.

"We have no idea whether these chickens were killed and eaten or slaughtered," he said.

Meanwhile, Public Health Minister Sudarat Keyuraphan said her ministry was monitoring the condition of three hospital patients in central Thailand. Test results to determine if the patients had bird flu were expected in a few days.

Vietnam is the only country with confirmed cases this year of bird flu in people.

Thai officials have consistently said thousands of local chickens have died of bird cholera and respiratory disease. Some farmers, however, have accused the government of a cover-up.

Still, authorities have killed about 1 million chickens since November and will destroy millions more to control the cholera and respiratory disease outbreaks, Deputy Agriculture Minister Newin Chidchob said. "I think it will be in the millions, though I can't say how many millions," he said.

Cambodia, which has temporarily banned imports of Vietnamese poultry, said Wednesday it will destroy 159,000 duck eggs seized from traders who smuggled them illegally from Vietnam.

In Hong Kong, a dead falcon tested positive for bird flu Wednesday, prompting officials to step up surveillance at local chicken farms, although they said the public was in no danger.

The peregrine falcon had the deadly H5N1 virus, which crossed over from chickens to humans in Hong Kong in 1997 and killed six people.

A WHO team plus six scientists from the U.S. Centers for Disease Control (search) were in Vietnam investigating how the H5N1 virus jumped from poultry to people there, Dietz said.

So far, there has been no evidence of person-to-person transmission. But health officials have warned that if the avian virus mutates to allow human transmission, it could make the disease a bigger health crisis than SARS, which killed nearly 800 people worldwide last year.

The spread of bird flu, along with the re-emergence of severe acute respiratory syndrome -- with three recent cases confirmed in China -- has put Asia on a region-wide health alert.

It is the first such bird flu epidemic in Japan since 1925, and the first ever documented in Vietnam and South Korea.