French authorities say they cannot rule out bird flu after 4,000 chickens died on a French farm on Saturday.
There are concerns worldwide about the spread of bird flu
Officials have carried out tests on the dead birds found in Sarrey in the northeast Haute-Marne region, but the results are not expected until Tuesday.
The farmer says his flock of 7,000 chickens appeared healthy on Saturday morning, but by evening 4,000 had died.
France is the biggest poultry producer in Western Europe, breeding some 900m chickens, ducks and geese every year.
Poultry producers fear their Christmas sales could be badly affected if the latest cases are confirmed as bird flu, says the BBC's Caroline Wyatt in Paris.
"In the absence of another explanation, we cannot rule out the theory of avian flu," an official of the French Veterinary Services Directorate told France-Info radio.
"We are obliged to speak of suspicion," he said.
The samples are now being analysed at France's main testing laboratory in Ploufrangon.
The French authorities have sealed off the farm as a precaution.
In March, the farm itself was used as the site of a government exercise on how to contain avian flu - when France was on high alert after migrating birds carrying the virulent strain H5N1 infected one poultry farm.
Millions of birds have died or been destroyed as a result of outbreaks in dozens of countries since the H5N1 bird flu strain emerged in South-East Asia in 2003, before spreading to Europe and Africa.