The bird flu or chicken flu that has decimated Vietnam's poultry flock and killed five people may have reached Thailand and Thai officials are investigating whether 3 people are infected and suffering from the effects of the H5N1 avian influenza.

Thailand is among the world's largest exporters of chickens in the world, and is a key supplier of chicken meat for Europe.

The World Health Organization has said it is very concerned that the avian influenza virus could mutate into a far more lethal form as it is currently spreading around the Asian region.

Japan, who is Thailand's largest market for chickens, has now banned poultry imports from Thailand, saying it accepts that bird flu is not yet confirmed in Thailand, but Japan can not rule out the risk of eventual infection.

Thailand's Government has for days insisted that the illness plaguing the Thai chickens, sparking a cull of millions of birds, is not bird flu or chicken flu, but Fowl Cholera and Bronchitis.

But on Thursday, Thailand's health ministry acknowledged it was investigating whether 3 Thai people, including a seven-year-old boy and a chicken farmer could be suffering the human form of the Bird Flu disease.

Japan, then moved quickly to ban all imports of Thai chicken meat, saying it was a precautionary step to ensure food safety in Japan.

Meanwhile, the WHO (World Health Organization) spokesman in Vietnam, Bob Dietz told the news agency AFP that the spread of the avian influenza virus across such a large part of Asia was a reason for growing concern. The more widespread the Bird Flu becomes, the more chance there is that it could mutate to other forms, he added.

Thai critics have accused the government of Thailand of a cover-up of Chicken Flu in the country. Despite the Thai governments reassurances, many Thais have chosen not to eat chicken for their Chinese New Year celebrations, says the BBC's Kylie Morris, in Bangkok Thailand.