The European Union has joined Japan in banning imports of Thai poultry amid growing international alarm over the spread of bird flu.
Millions of birds have died or been culled across Asia
Thailand has confirmed its first two cases of avian flu in humans, and a Thai man tested for the virus has died.
Bird flu has affected poultry in Japan, Taiwan, South Korea - and also Vietnam, where it has killed five people.
The World Health Organisation warned that the latest outbreak could mutate and become more dangerous to humans.
Bird flu has been ravaging Asian flocks, but has so far only jumped from birds to humans in a handful of cases.
A 56-year-old Thai chicken butcher - one of six people being tested for bird flu in Thailand - died of pneumonia, Thai government officials said.
Bangkok had already confirmed that two boys had contracted the virus after touching carcasses of infected poultry - one boy in Suphanburi and one in Kanchanaburi province.
BIRD FLU ALERT
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 authorities say anyone suffering from fever and bronchitis after having been in contact with poultry should seek urgent medical attention.
Following the announcement, the European Union declared it was banning poultry imports from Thailand, after Japan took a similar step on Thursday.
The EU ban - on all poultry and poultry products slaughtered after 1 January - takes immediate effect.
Imports of steamed poultry heated to 70 degrees C can continue as this process kills the virus.
Chicken rearing is a major industry for Thailand, one of the world's largest chicken exporters.
The two markets account for the majority of Thailand's annual $1.3bn of chicken export earnings.
Earlier reports that Thailand was halting all chicken exports were later denied by deputy agriculture minister Newin Chidchob.
The latest bird flu outbreak has been spreading across Asia for weeks, but until Thailand's announcement, it was only known to have jumped to humans in Vietnam.
BIRD FLU TIMELINE
Nov 2003 - Thailand deals with what it calls chicken cholera
15 Dec - S Korea confirms bird flu outbreak
9 Jan 2004- UN sends help to Vietnam after bird flu outbreak
11 Jan - First of five Vietnamese deaths confirmed as bird flu
13 Jan - Japan confirms bird flu outbreak
15 Jan - Taiwan announces different strain of bird flu
21 Jan - Laos reports suspected chicken cholera
23 Jan - Thailand confirms first human cases of bird flu
23 Jan - Cambodia detects first case in chickens
WHO spokesman Bob Dietz, speaking in Vietnam, said that it could become more of a threat to humans as it spread because its form may alter.
"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," Mr Dietz said.
The Lancet medical journal also issued a dire warning on Friday, saying standard vaccines would be useless against the virus if it started spreading through humans.
At present, there is no evidence to suggest it can be passed on from one person to another.
Thailand's public health ministry issued the following advice for people to protect themselves against the 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
- Different species of poultry should be separated and their coops kept clean.