The poultry farm was found to be infected on Wednesday with the H5N1 bird flu virus - which is potentially fatal to humans - in a new round of testing of farms, agriculture officials said, with another nine farms already declared to have the disease.
Early on Wednesday, agriculture officials and troops had culled 700,000 birds from 1.28 million to be killed over the next few days, the agriculture ministry said.
YTN cable news television showed soldiers bagging dead birds for burial, as herds of ducklings and chicks were forced to march towards a huge ditch on one farm where they were to be buried alive.
Dangerous to humans
The virus was first confirmed at a farm in Umseong, 130 kms
southeast of Seoul, on 15 December. This week it jumped the quarantine zone around the farm and spread to others, notably those in the South Cheollar province which supplies nearly half of the country's ducks.
The H5N1 virus is considered potentially dangerous to humans and claimed six lives in Hong Kong in 1997. But officials from South Korea's National Institute of Health have suggested this strain could be a variant which poses no harm to humans.
Nonetheless, poultry consumption has plunged, sending chicken prices plummeting across the country. Chicken exports have also slowed with China and Japan banning the import of poultry or bird products from South Korea.