Public health workers began a cull of roughly 200,000 chickens Friday morning at a farm in Okayama Prefecture, western Japan, where an outbreak of highly pathogenic H5 bird flu was confirmed.
The prefectural government said the cull would take four days, with roughly 50,000 birds killed each day. The dead hens will be incinerated.
Self-Defense Forces personnel and local government staff are taking part in the work.
Authorities have banned farmers moving chickens and eggs at six farms within 3 km of the farm in Kasaoka, and 15 other farms within 10 km have been ordered not to ship their products.
The 21 farms account for nearly 1 million birds in all.
It is the fourth case of avian influenza detected at a poultry operation in Japan this winter.
The previous three cases this winter have resulted in the killing of thousands of chickens, ranging from 4,000 to 42,000 in each case.
Late Thursday night, Prime Minister Shinzo Abe ordered the agricultural ministry and other offices to implement epidemic prevention measures without delay.
The farm ministry said senior vice minister Toshiko Abe will travel to the prefecture to oversee the response.
The hatchery in Kasaoka filed a report to the local livestock health center Thursday morning after it found 28 birds dead from Wednesday.
A preliminary check confirmed infection in four of the dead birds and one live chicken. A further genetic test confirmed that it was the feared virus.
Okayama is the fourth-largest chicken egg producing prefecture nationwide. Its farms have around 10 million birds, figures from last February show.
The latest case comes after bird flu cases at two farms in Miyazaki Prefecture and another in Yamaguchi Prefecture in December.
In 2007, Okayama Prefecture culled around 10,000 chickens after a highly pathogenic virus was detected in the city of Takahashi.