Croatian authorities have started culling all poultry around a fish pond where the country's first bird flu case was confirmed, and police sealed off the area.
The officials said some 10,000 birds in around 1,000 farms and rural households would be killed in the next few days.
On Friday, scientists detected the H5 avian flu virus in wild swans found dead at a fish pond between Orahovica and Nasice, in the east of the country, and sent samples to Britain to determine whether the virus was the lethal H5N1 strain. The results are expected on Monday or Tuesday.
The Croatian Agriculture Ministry said it had banned export of poultry to EU countries. European Commission earlier said it was in preparations to enforce such a ban. Italy already banned imports of live poultry from Croatia, Romania and other Balkan countries.
The deadly H5N1 strain of bird flu has been detected in Romania, which shares the Danube waterway with Croatia, and in Turkey. Police blocked all roads leading to the pond, situated in a fertile plain with wheat and cornfields.
Meanwhile a duck found dead east of Stockholm has tested positive for avian flu, Sweden's National Veterinary Institute said last night, but it was too early to determine which strain of virus the bird had.
"The result of the virus tests which were carried out today on samples from ducks found in Eskilstuna show that one of them was infected with influenza," the institute said in a statement.
The institute said that at this time of year it is not unusual for 20-30 percent of ducks to carry influenza. Further tests are being conducted on samples from the duck, which was found on Friday, to determine whether the virus is of the H5 strain and whether it is highly or less pathogenic.
The Swedish Board of Agriculture has advised poultry farmers in the central and southern regions of Svealand and Gotaland to bring birds indoors if possible or feed them in protected areas.