South Korea's Ministry of Agriculture, Food and Rural Affairs and the Gyeonggi provincial government said officials are testing samples of three dead pigs from two farms in Paju, a city where the country's first case of the disease was confirmed Monday. Test results were expected to come out Friday night.
The two Paju farms, which raise more than 7,000 pigs combined, were also within 10 kilometers (6.2 miles) of a farm in the nearby town of Yeoncheon, where a second case of the disease was confirmed Tuesday, said Park Byeong-hong, an agriculture ministry official.
African swine fever is harmless to humans but highly contagious and fatal for pigs as there is no known cure. It has decimated herds in China and other Asian countries.
The ministry said quarantine officials were testing blood from pigs at some 100 farms within 10 kilometers (6 miles) of the infected farms in Paju and Yeoncheon, and that samples from 56 farms had come back negative.
About 6,300 farms in South Korea raise more than 11 million pigs. South Korean officials have said the next three weeks would be crucial for fighting the outbreak, considering the disease's incubation periods.
Officials have stepped up efforts to disinfect farms and vehicles. They imposed temporary bans on farms near the border from transporting their pigs to other areas and began inspections of some 200 slaughter houses, feed factories and artificial insemination facilities that deal with large numbers of pig farms across the country.
More traps and nets will be installed to capture wild boars that roam in and out of North Korea, which some experts see as a potential source of the outbreak in South Korea. South Korea's Defense Ministry has dispatched soldiers to support quarantine efforts and monitor areas along a river that flows through the border between the Koreas, searching for wild boars that may swim across from the North.
"It's crucial to strictly restrict the movement of people, cars and animals and also prevent (pigs) from contact with wild boars," Park said.
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, complicating efforts at preventing the North Korean outbreak from spreading to areas near the border.
South Korea's Unification Ministry, which deals with affairs with the North, said Friday that Pyongyang is continuing to ignore Seoul's calls for joint quarantine efforts to fight the disease.