HELSINKI (AP) — Norwegian authorities haven't been able to detect the cause behind an unexplained disease that is estimated to have killed dozens of dogs in the country in recent days, officials said Saturday.

The Norwegian Food Safety Authority said that it had been informed of another six cases of dogs falling ill, with two them already dead, all with the same symptoms of vomiting and bloody diarrhea.

The disease seemed "very serious for a dog. But we don't know yet whether this is contagious or just a series of individual cases," agency spokesman Ole-Herman Tronerud told public broadcaster NRK.

The majority of cases have been reported in and around the capital Oslo, but also in the cities of Bergen and Trondheim, and the northern Nordland municipality.

The Norwegian Veterinary Institute said Friday that it had detected two unspecified bacteria in autopsies, but was unable to clarify whether that was the cause of the outbreak.

"To have healthy and great Norwegian dogs die so quickly is naturally serious. It's a very special situation I haven't been involved in before," the institute's emergency and safety director, Jorun Jarp, said Friday.

In neighboring Sweden, the National Veterinary Institute said it had been swamped with questions from dog owners concerned over the spread of the disease across the border.

The institute said it currently had no information "to show that it is an infectious outbreak or that it would pose a risk to dogs in Sweden."