A plague of field voles in central Spain has led the authorities to resort to controlled burning of fields.

Farmers say they have never seen a vole plague like this one

The measure is only permitted by the European Union in exceptional cases.

The fires were started on Wednesday in the Valladolid area of Castilla-Leon region, to prevent the rodents devastating beet and potato crops.

Agricultural officials say they are not sure what caused the surge in vole numbers. Fields will also be burnt soon in Avila, Palencia and Segovia.

The farm union COAG says about 400,000 hectares of land (988,000 acres) is affected by the infestation, Reuters news agency reports.

Castilla-Leon's Agriculture Minister, Silvia Clemente, said agronomists, veterinarians and biologists had been asked to explain the plague but nobody really knew, she told Spain's Cadena Ser radio.

"There has never been a plague like the one we have now," she said.