Fresh cases of avian influenza were found in duck breeding farms in the South Cholla province, home to half of South Korea's eight-million ducks.
Since the first bird flu case was detected at a farm 80km southeast of Seoul, cases have been confirmed at eight more poultry farms across South Korea.
Agriculture ministry authorities said about half of the target of one million chickens and ducks had been slaughtered and buried.
"We will complete the slaughter today or tomorrow," Kim Dal-joong, senior agriculture ministry official, told reporters on Tuesday.
The ministry said in a statement that 13 suspected cases were being tested, including seven new overnight reports.
Authorities are disinfecting chicken farms to protect the country's 100 million chickens.
The government also sprayed zoos to protect rare birds from the avian flu.
Farmers have been told to stay away from places where migratory birds gather because wild fowl are one suspected source of the disease.
Test results awaited
The government is awaiting test results from the US to determine if the latest virus is similar to the H5N1 variant of avian influenza that killed six people in Hong Kong in 1997 and 1998.
Meanwhile, consumers have been avoiding duck and chicken - ignoring exhortations by government officials who have been eating poultry dishes on television almost every day.
In rare cases, bird flu is lethal to humans
"Like most bird flu types, this does not seem to affect humans," Kim Moon-shik, director general at National Institute
of Health, said.
Kim said almost 1000 people in the affected areas were tested but no one showed symptoms of the disease.
In rare cases, bird flu is lethal to humans, with a virus incubation period of four to five days.