Officials were planning to cull some 5,000 pigs raised at a farm in the town of Yeoncheon after the highly contagious disease was confirmed in tests of a dead pig, a South Korean agriculture ministry official said.
Officials culled nearly 4,000 pigs and stepped up quarantine efforts Tuesday after confirming the country's first case of the disease at the nearby city of Paju.
African swine fever is harmless to humans but highly contagious and fatal for pigs. It has decimated herds in China and other Asian countries and there is no known cure.
"I urge you to quickly respond and do whatever you can," he said.
"We will obviously cull the 4,700 pigs raised at the Yeoncheon farm and could cull more pigs at farms in neighboring areas if needed," said the ministry official, who declined to be named, citing office rules.
There are about 6,300 farms in South Korea that raise more than 11 million pigs.
North Korea in recent months has virtually scrapped all diplomatic activity and cooperation with South Korea amid a standstill in nuclear negotiations with the United States, and has ignored repeated South Korean calls for joint efforts to stem the spread of the disease.
South Korean investigators are tracing the source of the outbreak, and officials say it isn't immediately clear if the disease would have crossed from the North.
In other regions, wild boars are thought to have caused the virus to spread.
Seoul's Unification Ministry, which deals with inter-Korean affairs, said Wednesday that it notified the North of the outbreak in South Korea through a liaison office at a North Korean border town.
South Korea in past months has placed fences and traps around border area farms to prevent pigs from being infected by wild boars that roam in and out of North Korea. Government officials said it would be difficult for the animals to cross over barbed wire fences in the mine-scattered border zone, but they could swim across rivers.
South Korea's environment ministry said it plans to dispatch personnel to strengthen monitoring on movement of wild boars in border areas and install more traps and nets. The ministry said it asked local officials not to shoot the animals because gunshot sounds could cause them to scurry and cover more ground.