Bird flu cases across Japan are set to lead to the culling of a record 10-million-plus chickens and other birds.

The flu has spread with unprecedented speed since the season's first cases were confirmed in Kurashiki City, in western Japan's Okayama Prefecture, and in Atsuma Town, in Hokkaido in northern Japan, on October 28 last year.

The agriculture ministry says the number of affected farms and other locations hit a record 57 places in 23 prefectures with the addition of a poultry farm in Kawaminami Town, Miyazaki Prefecture in southwestern Japan, on Tuesday.

It also says the number of birds being culled in a single season will be confirmed at 10.08 million, exceeding 10 million for the first time.

The previous high was 9.87 million between the fall of 2020 and the spring of 2021.

Experts attribute the quick spread of infections to the earlier-than-usual arrival of migrating wild birds into Japan in late September. They said this resulted in an unprecedented scale of virus transmission.

They warn that the virus may remain among non-migratory birds in Japan until the spring.

Agriculture ministry officials are asking farm operators to ensure facilities are properly sanitized. They are also calling for repairs to facilities so that wild animals, which can carry the virus, are kept away from poultry.