The results of tests taken from the three birds, found in separate areas near the northern Greek cities of Thessaloniki and Katerini, were expected to be reported in two to eight days.

The deadly H5N1 strain of the virus, which has killed four people in neighbouring Turkey, has also forced other Balkan countries to cull thousands of poultry.

The European Union's executive Commission said it was taking precautionary measures in response to the Greek discovery.

"The Commission will adopt tomorrow (Friday) a safeguard measure to ensure increased biosecurity on poultry farms in the concerned areas and improved disease awareness of poultry owners and their families," the EU executive said in a statement.

"Movements of poultry from the affected area to other holdings for slaughter will be subjected to rigorous additional controls."

A spokesman for the Brussels body said farms within a 3 km (2 mile) radius of the swans would need to disinfect structures housing poultry.

Greece's Health Ministry said in a statement all relevant health services in the areas where the bird flu samples were found would be on a heightened alert.

"We remind people who come to contact with wild birds, dead or alive, such as hunters and wild animal care volunteers, that they must take all necessary protection and hygiene measures," it said.