Bird flu has spread to a sixth farm in South Korea, despite government efforts to contain the deadly virus by culling more than two million birds, officials have said.

A new outbreak has been discovered at a farm near the capital Seoul with some 133,000 chickens, the agriculture ministry said.

"Test results confirmed that breeding chickens at a farm in Iljuk Village were infected with high contagious bird flu virus," the ministry said in a statement.

Poultry at the farm, in Iljuk Village near Ansung City, some 90 kilometres (56 miles) southeast of Seoul, began dying on Tuesday and the case was reported to authorities on Friday. It is not yet known whether the virus is of the highly pathogenic H5N1 strain.

Officials immediately ordered the culling of the remaining chickens, and of 107,000 other birds at 28 farms within a three-kilometre radius of the affected farm.

They also declared a 10-kilometre quarantine zone, from which movement of birds and eggs will be prohibited.

It is the country´s sixth outbreak of avian influenza since November.

Officials have slaughtered 2.3 million birds at farms including those 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.