The rising number of bird flu cases in France is unusual but unlikely to turn into a crisis as seen in the United States this year that led to the culling of millions of poultry, the head of World Organisation for Animal Health (OIE) said.

France, the European Union's largest agricultural producer, has faced a series of diverse outbreaks of bird flu since a case of H5N1 was found in chickens in the southwest region of Dordogne on Nov. 24.

Four departments, or regions, out of 96 in mainland France are now concerned. Twelve outbreaks have been reported, involving three different strains of highly pathogenic avian influenza (bird flu): H5N1, H5N2 and H5N9. Some outbreaks still need to be precisely identified.

"It is an unprecedented situation to see the emergence of three different strains in such a short time," OIE Director General Bernard Vallat told Reuters in an interview.

Investigations were still ongoing but he said there could be two explanations. Either the strains were carried by migratory birds, or low pathogenic strains evolved into high pathogenic ones.

Vallat played down the situation, which has shaken France's major foie gras producing regions, coming just before demand peaks over the year-end holiday season.

"We are not in a situation like the one in the United States because the breeding methods are different and the strains less virulent," he said.

"Poultry farming in the departments hit is extremely active, notably for foie gras, but finally it did not spread that fast unlike other situations we have known where there were a lot of outbreaks," he said, referring to the United States.

More than 48 million chickens and turkeys have been culled in the United States since December 2014 because of bird flu. Egg prices reached all-time highs because of the losses and dozens of countries imposed total or partial bans on U.S. poultry and egg imports.

Eight countries have restricted imports of French poultry birds and products due to the recent outbreaks, including Japan which is France's biggest export market for foie gras.

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Vallat insisted that the H5N2 and H5N9 strains had never been found in humans and that the H5N1 strain found in France was different from the Asian one that caused many human deaths.

He also stressed bird flu cannot be transmitted by food.

"You can tell a consumer that there is no risk even if he eats a chicken or a sick duck," he said. "For the moment, no need to worry. There has never been a human case linked to the ingestion of eggs or poultry."