GENEVA, Jan 22: The World Health Organization expressed concern on Thursday that a strain of bird flu, which has so far killed five humans, has for the first time broken out simultaneously in several Asian countries and could mutate into a far more lethal form.

With the region still smarting from the devastation of last year's SARS epidemic, the appearance of H5N1 bird flu in several states was both unprecedented and put the entire region at risk, said Isabelle Nuttall, a doctor with WHO's contagious diseases department.

"What is worrisome is that for the first time incidences of bird flu are appearing simultaneously in several countries. We've never seen that before," she said.

Since December 15, cases of H5N1 have been confirmed among poultry in Japan, South Korea and Vietnam - the worst hit and the only place so far where human contamination has been confirmed. All five deaths occurred in Vietnam and 17 other people are being closely watched in hospital.

A sub-category, H5N2, has also been found in Taiwan. A question mark however still hung over Thailand - where the government was bitterly accused of a cover-up to protect its valuable poultry industry, the largest in Asia whose exports brought in 1.2 billion dollars last year.

The scare was already causing fallout as Japan, Thailand's largest poultry export market, announced Thursday a ban on imports of Thai chicken. France meanwhile decided to remove from the market all fresh meat imported from Thailand since January 1, while Brussels warned that the European Union - the second-biggest importer of Thai poultry - could ban Thai chicken imports if the outbreak is confirmed there.

Previous incidences of the bird flu have been confined to single countries, such as an outbreak in Hong Kong in 1997 or one in the Netherlands last year. WHO spokesman Bob Dietz in Hanoi warned that "although we have seen no evidence of human-to-human transmission the next step would be for that to occur."

All the Vietnamese cases were contaminated by direct contact with poultry. "It is impossible to predict a time or date for this but there are mounting opportunities for the virus to alter its form and begin affecting the human population," Dietz said.

Mass cullings and killings poultry flocks have been conducted, in both affected countries and Thailand despite its repeated denials of the disease.The Thai government said it would not know until Friday whether the three suspected cases there, including a chicken butcher and a seven-year-old boy, were infected with the bird flu.

However, Thai senator Nirun Phitakwatchara, who is also a doctor, insisted the boy's case had been confirmed and accused the government of a cover-up to protect its export revenue.

"We have for the moment received no official notification from the Thai government," Nuttall said. "We are aware of the press reports but we are not yet in a position to make any official statement."

Nuttall said the virus can be transmitted by migrating birds, but said it was not yet certain how it had spread to several countries at once. China - which is considered a hotbed for viruses and the origin of the Severe Acute Respiratory Syndrome (SARS) last year that, like bird flu, has been traced to Asia's popular live animal markets - has yet to report any outbreak of bird flu. But Hong Kong announced Wednesday that a wild falcon found dead near a chicken farm had tested positive for H5N1.-AFP