Japan has suspended imports of Thai chickens after Thailand said it was investigating possible cases of a human strain of bird flu.
The disease only affects people in close contact with live chickens
Three people in Thailand are being tested for avian influenza, which has killed at least five people in Vietnam.
Despite initially playing down fears of an outbreak, Thailand has issued advice on how people can protect themselves.
Japan, the major importer of Thailand's chicken, says the infection is not yet confirmed, but cannot be ruled out.
First jumped "species barrier" from bird to human in 1997
In humans, similar symptoms include fever, sore throat, and cough
Types known to infect humans are influenza A subtypes H5N1 and H9N2
The World Health Organization says it is concerned the virus could mutate into a far more lethal form as it spreads around the region.
At present the virus has only spread from live chickens to humans, and has infected only a small number of people.
Critics accused the Thai Government of a cover-up, after it had insisted that an illness that sparked the recent cull of a million chickens was not bird flu, but chicken cholera or bronchitis.
But on Thursday the public health ministry acknowledged it was investigating whether three people - including a seven-year-old boy and a chicken farmer - could be suffering the human form of the disease, virus subtype H5N1.
The ministry issued the following advice for people to protect themselves against the deadly flu:
- Eat chicken only when it is well-cooked, and only eat cooked eggs;
- Anyone developing fever, muscular aches and severe respiratory problems should report to health professionals;
- Farm workers should wash their hands thoroughly and anyone working with poultry should wear masks and gloves;
- The different species of poultry should be separated and their coops kept clean.
Japan moved quickly to place a temporary ban on imports of Thai chicken, saying it was a precautionary step to ensure food safety.
More than half of Thailand's exports go to Japan, and the news prompted a fall in the shares of chicken exporters of around 7% on the Thai stock exchange.
The European Union is another important market for Thailand. EU Health Commissioner David Byrne - currently in Thailand on an official visit - said he was monitoring the situation closely.
"I am increasingly concerned about this. I am evaluating the situation hour by hour as it develops," he told the BBC in an interview.
Japan, South Korea and Taiwan are all fighting outbreaks of bird flu but have reported no human cases.
WHO spokesman in Vietnam Bob Dietz told AFP news agency that the spread of the virus across such a large part of Asia was a reason for "growing concern".
"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," he was quoted as saying.
The main fear is that the virus could gain the ability to be spread from person to person, says the BBC's Kylie Morris, in Bangkok.
Nirun Phitakwatchara, a Thai senator and vocal critic of the government, told Reuters news agency that laboratory tests confirmed seven-year-old Virat Phrapong had the disease in Suphanburi province, 100 km (60 miles) north of Bangkok.
He accused Thai officials of trying to cover up the outbreak "for business and political reasons", according to the Associated Press news agency.
But Prime Minister Thaksin Shinawatra said it would be days before tests showed whether the
child was a victim of the virus.
Despite government reassurances - including recent TV pictures of ministers tucking into a chicken lunch - many Thais chose not to eat chicken for their Chinese New Year celebrations, says our correspondent.