The country has been free of the disease for eight months and was expected to soon recover its sales of fresh meat.
But Thailand's Agriculture and Cooperatives Ministry said yesterday that deaths in around 30 fowl two weeks ago had been caused by the bird flu virus.
The news is a setback after the country announced earlier this month that it hoped to be completely free of the virus in three years.
However Pornsri Laurujisawat, manager of the Thai broiler processing export association, told AP-Foodtechnology.com that the new outbreak would not damage exports.
In the wake of a ban on fresh poultry meat in place since early 2004, Thailand's poultry processors have been concentrating on cooked meat, and exports of this product are forecast to reach 350,000 tons this year, up from the 276,000 tonnes shipped in 2005.
It is still significantly less than the 543,000 tons of total poultry meat exported in 2003 but shows the country, one of the world's biggest chicken exporters, is on track to recover its sales.
It has already shipped 140,000 tonnes of chicken abroad during January to June, mostly to Japan and the EU.
Thailand was applauded by international organizations for its strong controls that prevented bird flu from recurring in recent months but farmers and an outgoing senator have accused the government of covering up the recent cases which occurred around two weeks ago.
Ministers tried to ease public concerns by saying that the virus had been found in an area that used to be plagued with the disease and not in a bird flu-free zone.
The government says the virus was unlikely to spread since fowl in the infected area had now been culled and it has implemented a total ban on fowl movements in the province and sealed borders.