As South Korea battles an outbreak of African swine fever (ASF), the destruction of some 47,000 pigs has led to the Imjin River, which runs through the demilitarised zone, turning blood red.

Key points: Blood from culled pigs washed into the Korean peninsula's Imjin River after heavy rains

Blood from culled pigs washed into the Korean peninsula's Imjin River after heavy rains The Imjin is the seventh-largest river in South Korea and it flows through the demilitarised zone

The Imjin is the seventh-largest river in South Korea and it flows through the demilitarised zone South Korea culled the pigs due to an outbreak of African swine flu

The strange colour is the result of the river being polluted with the blood of many of the slaughtered pigs. Heavy rains caused their blood to flow from a border burial site into a tributary of the Imjin.

South Korean authorities culled the pigs in an attempt to halt the spread of the disease, which is highly contagious and incurable, with a near zero survival rate for infected pigs.

It is not dangerous to humans though.

Still, the contamination of South Korea's seventh-largest river has prompted concerns it could aid the spread of ASF to other at-risk animals.

However, authorities dismissed such fears, saying the culled pigs were disinfected before being slaughtered.

They also said emergency steps had been taken to prevent further pollution of the river, which was the site of a battle during the Korean War.

The pig-culling operation was carried out over the weekend. The carcasses were said to have been left inside multiple trucks at a burial site near the inter-Korean border.

A delay in the production of plastic containers used in the burial meant it could not be carried out immediately.

North Korean link

ASF was only discovered in South Korea recently, and there was speculation it arrived via pigs crossing the heavily guarded demilitarised zone (DMZ) that separates the north and south even though South Korean officers at the zone had been authorised to and have shot dead wild boars trying to cross.

Despite the precautions, South Korea reported its first ASF case on September 17, with the total now at 13. There are about 6,700 pig farms in South Korea.

Much of Asia has been affected by the outbreak, including China, Vietnam and the Philippines.

In China, where pork is a staple of the diet, 1.2 million pigs have reportedly been culled. More than 5 million are believed to have been culled in Vietnam.

Mongolia, the Philippines and Laos have also culled tens of thousands of pigs.

The virus is yet to reach Australian shores but it is having an effect on the industry here, with "over-the-hook" pork prices rising from roughly $2.50 per kilogram last year to $3.50 per kilogram this year.