The mass slaughter took place at a poultry farm near the central city of Cheonan south of Seoul, the agriculture ministry said.

"We started culling 273,000 chickens and other poultry this morning within a 500 meter radius of the farm," a ministry official said.

The ministry blamed migrating wild geese for spreading the highly contagious virus, saying the strain was the same as one found among wild birds in northeastern China.

Officials have slaughtered some 1.2 million birds at farms near the southern cities of Iksan, Gimje and Asan after the country's first case of bird flu in almost three years was confirmed on November 25.

South Korea was hit hard by bird flu between December 2003 and March 2004, prompting the cull of 5.3 million poultry costing about one billion dollars.

Bird flu has killed more than 150 people worldwide since late 2003 and fears remain it may become a far more highly contagious disease that could trigger a deadly global pandemic.