The strain of the bird flu virus lethal to humans has been found in Croatia, the European Union has announced.
This is Croatia's first case of the deadly strain
The virus, also known as avian flu, was discovered in wild swans found dead at a pond in eastern Croatia last week.
After tests in the UK, it was confirmed to be the H5N1 strain that has killed at least 60 people in Asia since 2003.
Officials are braced for an outbreak of the lethal strain in the EU, after it was recently discovered in Turkey, Romania and Russia.
Veterinary expert Vladimir Savic repeated a plea to Croatian poultry producers to "take this seriously" and keep their animals indoors.
"The situation is serious for the whole of Europe until a strategy is found, so I cannot say for how long the poultry will have to be kept indoors," he said, the Reuters news agency reported.
Six dead swans were found in the nature park of Zdenci, some 200km (124 miles) east of the capital, Zagreb, last week.
Experts later found more than 10 dead swans, believed to have belonged to the same flock, in a nearby fish pond.
The Croatian authorities have since disinfected and quarantined areas around the sites and killed tens of thousands of poultry in the area.
A EU spokesman said a precautionary ban on imports of live poultry, wild birds and feathers from Croatia, imposed over the weekend, remained in force.
The EU announced on Tuesday that it would ban imports of live wild birds and impose stricter rules on the private ownership of pet birds, after a parrot died of the H5N1 strain while in quarantine in the UK.
As the ban was announced, German officials said two geese had tested positive for the flu in initial checks.
Further tests are to be carried out to confirm the virus and to establish whether it is the deadly strain.
In Asia, China has reported its third major outbreak of bird flu in two weeks.
BIRD FLU OUTBREAKS IN 2005 (H5N1 STRAIN)
The H5N1 strain remained largely in South-East Asia until this summer, when Russia and Kazakhstan both reported outbreaks
Scientists fear it may be carried by migrating birds to Europe and Africa but say it is hard to prove a direct link with bird migration
UK case discovered in quarantine, so disease-free status unaffected