South Korea's quarantine agency said Wednesday at least two strains of highly pathogenic avian influenza have entered the nation via migratory birds this winter.

The Animal and Plant Quarantine Agency announced the results of DNA analysis into viruses detected at two duck farms in Gochang and Yeongam, both in Jeolla Province.

The virus discovered in Gochang on Nov. 19 was the H5N6 bird flu, which arose from the reassortment between a highly pathogenic H5N8 that spread in Europe in late 2016 and a low-pathogenic N6 virus, the APQA said.

It was identical to the one found on the southern island of Jeju from Nov. 27-Dec. 2.

The strain discovered in Yeongam on Dec. 10 was also H5N6, but its viral sequence had a slightly different pattern from the one in Gochang, it noted.

Highly pathogenic avian influenza is a variety of viruses that cause severe disease in birds and result in high death rates, according to the World Health Organization.

On Wednesday, the agriculture ministry said the H5 strain AI was newly discovered at a farm in Yeongam, which had received baby ducks from the affected farm about 40 days ago.

The latest outbreak took place at a farm with 31,300 ducks, located 11 kilometers from the affected farm that had been under monitoring since last week.

With the discovery, quarantine officials issued a "standstill" order for poultry farms to ban the movement of farmers, vehicles and equipment for 24 hours.

The ministry said it plans to announce whether the latest outbreak involves a highly pathogenic strain as early as Thursday.

Also, the government confirmed that it was the highly pathogenic H6N6 strain discovered in wild bird droppings on Dec. 13 in Yongin, south of Seoul.

It is the fourth case discovered in bird excretions in two months. One case occured in mid-November in Jeolla Province, and two others were discovered on Jeju Island in late November.

South Korea has been stepping up quarantine measures in duck farms in its southwestern region as outbreaks of highly pathogenic bird flu caused alarm in the major duck producing area.

Quarantine officials culled 76,000 ducks at five duck farms located within a 3-kilometer radius of the affected site earlier this month as Jeolla Province is the nation's largest duck producing region.

Last year, South Korea slaughtered more than 30 million birds to contain the worst outbreak of bird flu to have hit the country. (Yonhap)