The agriculture ministry confirmed the fourth case of swine fever in the town of Paju, near the border with North Korea, after reporting the county's third case late on Monday (23 September).

Since South Korea's first outbreak of African swine fever was discovered on 17 September, more than 15,000 pigs have been culled, according to data from the agriculture ministry, or about 0.1 percent of the country's pig population of 12.3 million pigs.

South Korea, Asia's fourth-largest economy, has raised its animal disease alert to the highest level following the outbreak and ramped up disinfection measures to try to keep the virus from spreading further.

Kim Hyeon-soo, the country's agriculture minister, said, however, that South Korea's quarantine controls had not been strong enough.

He said the government has imposed a transport ban for pig farms across the country for 48 hours from today noon (0300 GMT), but might lengthen the ban.

Kim also said the country would increase disinfection efforts in areas bordering North Korea, including in zones controlled by the military.

African swine fever was first discovered in North Korea in May, and it has continued spreading there as well, a South Korean lawmaker said today, despite the impoverished country's efforts to cull pigs and ban distribution of pork.

After North Korea's first outbreak of the highly contagious virus four months ago, it imported disinfectants from overseas, but it has not been able to contain the disease, the lawmaker, Lee Eun-jae, told reporters after being briefed on the matter by the South's spy agency on Tuesday.

In North Korea, which faces chronic food shortages, swine fever has killed all pigs in North Pyongan province, the Yonhap News Agency reported, citing South Korea's intelligence agency.

South Korea has yet to determine how or from where African swine fever entered the country, although it is conducting an investigation. It has also proposed to work together with North Korea to keep the virus from spreading in both countries, but Pyongyang has no responded yet, according to the Unification Ministry in South Korea.

The virus has now spread throughout Asia, including Vietnam, Laos and the Philippines - since first being detected in China in mid-2018, resulting in large-scale culls and lower output of pork.