The Ministry for Agriculture, Food and Rural Affairs said preliminary tests conducted on birds at the farms in Naju and Gangji showed they probably were infected with a strain of the avian influenza, reported the KoreaHerald.

While definitive test results will come out as early as Friday, the government issued a "yellow" alert, dispatched quarantine teams to the sites and has started to restrict the movement of animals, people and vehicles, it added.

Seoul has a four-tiered alert system with "yellow" being the third-highest level below "red" and "orange." At normal times a "blue" readiness posture is maintained.

The ministry also said as part of its precautionary measures, the 14,300 birds raised at the two farms will be culled, and starting from midnight, a "standstill" order will go into effect for the whole of South Jeolla Province and Gwangju for 24 hours, to arrest the possible spread of the bird flu.

"Actions taken are in accordance with the standard operating manual set up to deal with bird flu outbreaks," said Lee Jun-won, deputy agriculture minister.

He said more stringent measures are being taken now, compared to the past, because the government wanted to contain the AI at the outset.

Compared to the past when farmers reported sick birds to authorities, the latest case came to light after government quarantine inspectors detected signs of sick birds on Tuesday. The country has been keeping constant watch on bird farms since March.

Besides culling birds at the duck farms and isolating the region, the government will order the general decontamination of all poultry farms across the country, and strengthen the monitoring of areas that are most vulnerable to bird flu.

Since the winter of 2003, South Korea has been hit by five separate bird flu outbreaks with the last one lasting from Sept. 24, 2014, to June 10 of this year. The last outbreak caused damages totaling around 234.1 billion won ($199.3 million).