South Korea's Agriculture Ministry confirmed the country's fourth case of African swine fever on Tuesday at a farm in Paju, near the inter-Korean border.
This comes a day after the third case was confirmed in Gimpo, the first case south of the Hangang River.
The government is ramping up efforts to prevent any further spread, including culling pigs within a three kilometer radius of the affected farms.
Local authorities are also inspecting waterways that connect to North Korea.
Starting next month, South Korea's Rural Development Administration will work with Vietnam to discover the route of the spread as preliminary research toward a vaccine.
African swine fever currently has no vaccine, and is fatal to pigs, but harmless to humans.