Two of 17 European bison released this spring in Romania's Carpathian mountains where they had become extinct due to poaching have died of bluetongue disease, the national veterinary agency said Monday.

They were found dead in the wild zone of the Tarcu mountains, where 17 bison from Sweden, Germany, Switzerland and Italy were released in May in one of the biggest such operations in Europe.

"According to the tests we have conducted, their deaths were due to the bluetongue disease," said Radu Tataru, the spokesman for the veterinary agency in the southwestern Caras-Severin region.

"But we are awaiting the results from a laboratory in Bucharest for the official confirmation," he added.

The bison had disappeared from the Tarcu mountains about 200 years ago.

The European bison has recovered to a population of more than 5,000 after going extinct in the wild in the 1900s. But only 3,400 of them live in free or semi-free herds.

Decimated by hunters and the loss of their habitat, the European bison bounced back thanks to a large-scale breeding programme of the last survivors in captivity, whose offspring were reintroduced to the wild in areas of central and eastern Europe.

Bluetongue is a non-contagious, insect-borne viral disease that can affect sheep and less frequently cattle and other animals. Symptoms include high fever and swelling of the tongue and lips but mortality is relatively low.