Agriculture Minister Michel Barnier said the threat level from bird flu would be raised from "moderate" to "high" following the results of the tests.

The national laboratory of the French agency for animal health has confirmed the presence of the H5N1 highly pathogenic virus on the three swans found dead in a pond near the town of Assenoncourt, the ministry said.

France had stepped up surveillance after several cases of the H5N1 virus were discovered in Germany and the Czech Republic last month.