South Korea stepped up quarantine measures Saturday to stop the spread of bird flu and launched an investigation into the possibility that the deadly animal disease might have been caused by migratory birds, health authorities said.

The Animal and Plant Quarantine Agency on Friday found the flock of about 1,000 ducks dead in a reservoir in North Jeolla Province, and began an investigation to confirm a possible link to aviation influenza (AI), according to the provincial government.

The investigation comes after bird flu virus was found in thousands of ducks at a farm in Gochang, a nearby village in the same province.

It was the first case of AI since May 2011 in South Korea. More than 3 million poultry were slaughtered at the time in connection with the virus.

Later Saturday, the Ministry of Agriculture, Food and Rural Affairs issued an order in Jeolla Province and Gwangju city, 329 km south of Seoul, to temporarily prevent poultry, livestock raisers and their vehicles from leaving the areas on Sunday and Monday.

The order was issued for the first time in accordance with the quarantine guidelines for AI.

"The government issued the order limiting movement of related people and vehicles to carry out thorough sterilization and quarantine," Vice Health Minister Yeo In-hong said in a briefing at the Sejong government complex in central South Korea.

If AI is confirmed, it would be the first case of the virus in migratory birds in South Korea, authorities said, adding that such a result would have major ramifications.

Another flock of farm ducks is likely to have been infected with a highly virulent strain of the bird flu in the same region, the agriculture ministry said, adding that it is investigating for a possible link to the other cases.

The government has ordered a cull of some 6,500 farm ducks in North Jeolla Province's Buan village as part of preventive measures, a ministry official said.

"An investigation is under way to find a possible connection among the infection cases in Gochang and Buan farms and in the wild," the official said, hinting at the possibility that the AI-infected wild ducks spread the virus while flying over duck farms in the region.

The ministry will set up some 150 disinfection and quarantine posts in the region to prevent further infection, the official said. (Yonhap News)