Sweden confirmed at least one case of bird flu late Saturday, but said it was not yet known if it was the deadly strain that has swept through Asia. National Veterinary Institute spokeswoman Gunilla Hallgren said it could take days to determine whether it was the H5N1 strain of bird flu that killed four ducks found dead on Friday in Eskilstuna, about 100 kilometers (60 miles) west of Stockholm. "We know one duck has bird influenza, but we won't be able to say until tomorrow (Sunday) whether it is the H5 virus or if it is aggressive or not," Hallgren said. Institute officials said the findings so far were not necessarily a cause for concern. "It could very well be a mild flu virus, which is very common with such birds," institute technician Berndt Klingeborn said. "During this time of year, it isn't unusual that between 20 percent and 30 percent carry the flu virus." The H5N1 strain has devastated poultry stocks across Asia and killed 61 people in the last two years. The strain has recently been found in birds in Russia, Turkey and Romania.