Spanish authorities have slaughtered more than 130,000 hens after an outbreak of bird flu was detected at an intensive farming operation in the northern region of Castile and Leon, local officials said.

An industrial loader scooped up chicken carcasses by the thousand and dumped them into an open-topped truck, which took them to a burn site outside the farm, images provided by Greenpeace showed on Friday.

"The appropriate measures of slaughtering and disposing of the remains of all the animals have been taken on the farm, which remains isolated," the local veterinarians' association said in a statement on Thursday.

Authorities have imposed an isolation area of 10 kilometres around the farm, which lies just outside the village of Iscar in the province of Valladolid, roughly 130 km (81 miles) northwest of Madrid, but say there is little danger to humans.

"The possibility of transfer to humans is extraordinarily rare and therefore not a matter of great concern to us," Castile and Leon's regional government spokesman Carlos Fernandez Carriedo said.

Outbreaks of the virus, which is often transmitted by wild migratory birds to domestic animals, have been reported in France, Germany, Britain, the Netherlands and Serbia.

Spain has detected five smaller outbreaks in farms and a dozen in wild populations so far this year, according to the Agriculture Ministry.

A spokesman for the Spanish veterinarians association said it was unsurprising the virus had made it to Spain as the Iberian peninsula is on the path of many migratory birds.

But Greenpeace said industrial agricultural practices amplified the risk of the disease spreading, fuelling an ongoing debate in Spain about the economic and social costs of intensive farming.

"It is urgent to put an end to this destructive model that is jeopardizing the health of the planet and people," it said.