"As a preventive measure such things (exterminating flies) are probably needed to ensure the safest measures," said Hiroshi Takimoto, who heads the ministry´s office of infectious disease information.

Humans are normally only infected by contact with large concentrations of the bird flu virus and it is "scientifically impossible" for people to be infected by flies, Takimoto said.

The H5N1 bird flu strain was found in flies caught last March near a poultry farm in Kyoto in western Japan that had seen an outbreak of the virus the previous month, he said.

It was likely that this was the first time anywhere that the transmission of the bird flu virus to flies had been confirmed, he added.

In December, the Health Ministry said that at least one person had been infected with the virus after the Kyoto poultry farm outbreak -- the first human case of the virus in Japan.

The ministry said at the time that four others had also probably been infected, but added that none of the five had developed any symptoms of bird influenza and that there was no chance that they would infect others.

Thirteen people in Vietnam have died in the latest bird flu outbreaks there, which came a year after the highly contagious virus arrived in Asia.

The World Health Organization and health officials around the globe fear that the H5N1 strain might mutate into a lethal new virus that could spread rapidly among humans.