TOKYO - Japanese authorities on Saturday confirmed the country's fourth outbreak of the virulent H5N1 strain of the bird flu virus at a poultry farm in the country's south.

About two dozen chickens were found dead at the farm in Shintomi, southwestern Miyazaki state, last month. The birds had been infected with the H5N1 strain deadly to humans, the Agricultural Ministry said Saturday.

The case marks Japan's fourth H5N1 outbreak incident this year and the third to hit poultry farms in Miyazaki, Japan's largest chicken-producing region.

Officials began slaughtering the approximately 93,000 chickens at the Shintomi farm earlier this week.

The farm also has been quarantined, and chicken ranchers within a six-mile radius are banned from transporting poultry or eggs out of the area while officials check that the infection has not spread.

The H5N1 virus has killed or prompted the slaughter of millions of birds across Asia since late 2003, and caused the deaths of at least 164 people worldwide, according to the World Health Organization.

Japan has confirmed only one human H5N1 infection, and no human deaths.

Bird flu remains hard for humans to catch, but international experts fear it may mutate into a form that could spread easily between humans and potentially kill millions around the world.