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A mystery illness has killed a number of dogs in Norway over the past few days, officials say.

The Norwegian Food Safety Authority said up to 40 dogs had fallen ill with vomiting and diarrhoea, and several later died.

The first cases were in the capital Oslo, and others have since been reported in at least 13 other towns.

The Veterinary Institute said it was unclear if two suspect bacteria types found in autopsies were the cause.

Norwegian national broadcaster NRK reports that a total of 25 dogs have died from the condition. However, this has not been confirmed by the Norwegian Food Safety Authority.

A dog in neighbouring Sweden has also been admitted to veterinary hospital after appearing at a show in the Norwegian city of Trondheim, according to Swedish newspaper GT.

The Norwegian Veterinary Institute said its pathologists found that three dogs showed signs of a bloody gut, and that it had ruled out common causes like salmonella and rat poison.

Food safety authority spokesman Ole-Herman Tronerud told NRK that the illness seemed "very serious for a dog. But we don't know yet whether this is contagious or just a series of individual cases".

The agency has advised dog owners to keep animals on a lead in order to avoid contact with other canines.