Hungary says a new outbreak of the bird flu strain that can kill humans has been detected on a farm in the south of the country.
It is the first outbreak of bird flu in the EU this year
Tests were carried out after an abnormally high mortality rate was reported in a flock of 3,000 geese. The flock has since been culled.
Samples are being sent to the EU's approved laboratory in the UK to confirm the H5N1 strain of bird flu.
Croatia reacted by immediately banning poultry imports from Hungary.
The virus first appeared in Hungary in February last year in wild geese, swans and domestic poultry.
This is the first reported outbreak within the EU this year.
"The European Commission has been informed by the Hungarian authorities today of an outbreak of highly pathogenic avian influenza in Csongrad County, south-east Hungary," the EU Commission said in a statement.
The Hungarian authorities are also enforcing EU rules requiring a 3-km (two-mile) protection zone and 10-km surveillance zone around the infected farm.
The situation will be reviewed at a meeting of EU experts on Friday.
Since it was first detected in 2003, the strain has killed around 130 people, mostly in Asia.
Scientists fear it could combine with a human flu virus to mutate into a disease similar to the Spanish flu that killed millions of people after World War I.