South Korea continued quarantine efforts Tuesday as avian influenza was showing signs of spreading wider.

As of Tuesday morning, the virulent H5N8 strain of bird flu has been identified at 25 poultry farms, up from 17 farms the previous year, according to the Ministry of Agriculture, Food and Rural Affairs.

Two additional suspected cases have been reported overnight at a duck farm in Jincheon, North Chungcheong Province, and a chicken farm in Pyeongtaek, just south of Seoul in Gyeonggi Province.

The two new cases put the total number of not-yet-confirmed case of bird flu at 19.

More than 1.5 million birds have been culled since the first outbreak on Jan. 16 of the highly pathogenic H5N8 virus was reported at a duck farm in Gochang, 300 kilometers southwest of Seoul. Some 530,000 additional poultry, mostly ducks and chickens, are also set to be slaughtered.

The two latest suspected cases of AI came shortly after the government placed a 12-hour lockdown at all poultry farms in three provinces, including South Chungcheong Province, creating what it called a window of opportunity to simultaneously sterilize infection sources, including temporary homes for migratory birds.

"The government has mobilized about 190 vehicles and 3,800 workers to simultaneously sterilize poultry farms, farming facilities and areas that serve as winter homes to migratory birds," Deputy Agriculture Minister Lee Joon-won told a press briefing.

The government has pointed to migratory birds as the original source of bird flu after the H5N8 virus was identified in a group of wild ducks found dead Jan. 17 at a reservoir near a duck farm that reported the first outbreak of AI. The highly pathogenic strain of bird flu has since been identified in 13 other cases involving wild birds, the deputy minister said.

The 12-hour lockdown in three provinces, including Gyeonggi and North Chungcheong provinces, was lifted as of 6 p.m. Monday.

Ministry officials have said the H5N8 strain, unlike a few other strains of bird flu, posed no immediate threats to humans, noting no human infection of the strain has been reported. (Yonhap News)