The army has been disinfecting cars near the outbreak
South Korean officials say they have confirmed two new cases of foot-and-mouth disease at pig farms at Anseong county, about 100 kilometres (60 miles) south of the capital, Seoul.
The two cases are at farms close to the original source of the outbreak of the disease, which was first confirmed earlier this month.
The news is a blow to Agriculture Ministry officials, who have played down worries about the disease as tens of thousands of foreign visitors prepare to descend on South Korea for the World Cup football finals.
Officials had been saying that if no new cases were confirmed this week they could confidently announce that South Korea was safe from the outbreak.
The two new farms with affected pigs fell just outside a three-kilometre preventive cull zone set up around the original outbreak.
Nearly 2,000 pigs at the two farms are now being slaughtered and fumigation efforts are being stepped up.
Since foot-and-mouth was first confirmed earlier this month, 92,000 animals have been killed to prevent the disease from spreading.
The authorities had established a 10-kilometre quarantine zone around the site of the first outbreak and banned movements of livestock.
Two-thirds of the country's livestock markets have been closed.
Highly contagious
Foot-and-mouth is highly contagious. While it is harmless to humans, it can be deadly to cloven-hoofed animals.
The authorities say the outbreak will not affect preparations for the World Cup, as the cases are in rural areas, well away from the stadiums where matches will be played.
Experts believe the outbreak is different from the foot-and-mouth epidemic that swept across Britain last year and led to the slaughter of millions of animals.
They say the number of confirmed cases of foot-and-mouth in South Korea is far lower and the outbreak has affected just one small part of the country.