The bird flu outbreak has spread from North Jeolla Province to the Chungcheong provinces and Busan.
The agriculture ministry on Thursday said the H5N8 virus was the likely cause of death of three Baikal teals found dead in Seocheon, South Chungcheong Province on Tuesday.
A black coot and a black-headed gull found dead on Eulsuk Island along the Nakdong River in Busan on Thursday afternoon also tested positive for bird flu.
An ministry official said it is very likely that these migratory birds were infected with the virus.
The fresh outbreak started in North Jeolla Province last week, but the new cases this week in South Chungcheong Province and Busan lie well beyond the 10 km quarantine radius the government declared, suggesting the virus could spread nationwide.