The World Health Organization fears that the bird flu which has broken out in several Asian countries could mutate and become more dangerous.
The disease is linked to close contact with live chickens
WHO spokesman Bob Dietz said in Vietnam that it could become more of a threat to humans as it spreads.
Bird flu has killed five people in Vietnam and infected millions of chickens across Asia.
Thai Prime Minister Thaksin Shinawatra said it was "highly likely" that people had caught the disease in his country.
He said test results from five suspected cases would be announced on Friday.
Japan has suspended imports of chicken from Thailand.
Cases of the disease have been detected in Japan, Taiwan, South Korea and Vietnam.
"The more widespread it becomes the more chance there is that it could alter its form," said Mr Dietz.
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
"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."
Humans who have caught bird flu are thought to have been directly infected by live chickens. Only a small number of people have so far been infected.
But scientists worry about the possibility of human-to-human transmission, the BBC's Kylie Morris reports from Bangkok.
Japan suspended imports of Thai chickens following news that three people in Thailand were being tested for bird flu.
Critics accused the Thai Government of a cover-up when it suggested that an illness which 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 a number of 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.
More than half of Thailand's exports go to Japan and the flu reports prompted a fall in the shares of chicken exporters of around 7% on the Thai stock exchange.
Nirun Phitakwatchara, a Thai senator and vocal critic of the government, has accused Thai officials of trying to cover up the outbreak "for business and political reasons", according to the Associated Press news agency.
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 in Bangkok.