Up to 40,000 poultry are due to be slaughtered after bird flu was found responsible for the deaths of at least 25,000 turkeys in eastern Poland, according to local media and authorities.

Thursday's mass slaughter of birds comes in the Lubartow area, an important poultry farming region, Polish veterinary authorities said.

It was not clear how the bird flu virus found its way to the farms.

Poland is Europe's largest poultry producer, according to data from Eurostat, and last had an outbreak of bird flu in 2017.

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Andrzej Danielak, president of the Polish Association of Breeders and Poultry Producers, said three farms might be affected, with up to 350,000 birds at risk in a three-kilometre (1.9-mile) radius.

"Veterinary services are implementing virus eradication procedures in this situation," local authorities in Lubartow county said in a press release issued on Tuesday, adding that the virus was a subtype of the highly pathogenic H5N8 bird flu that can also threaten people.

The H5N8 strain has not yet infected any humans anywhere in the world to date, according to the United Kingdom's National Health Service.

The authorities said crisis meetings had been held, while footage from private broadcaster Polsat showed police cars blocking a road in the area.

Poland produces about 17 percent of the European Union's poultry, and exported more than a million tonnes of bird meat in the first half of 2019 - mostly to Germany, the Netherlands, the UK and France, according to the Dutch Ministry of Agriculture, Nature and Food Quality.