SEOUL - A third case of bird flu has been discovered in southwestern South Korea just as officials have completed culling hundreds of thousands of poultry from two earlier outbreaks.

Last month South Korea confirmed its first cases of the H5N1 strain in about three years, saying the virus had been found at two poultry farms close to each other in the North Cholla province.

The fresh case emerged after South Korea completed culling all 760,000 poultry near the two farms, raising concerns that quarantine measures had failed to control the outbreak.

"The new case could have nothing to do with the first two cases. We cannot say the virus has spread through the country," said an official at quarantine authorities who declined to be named.

The third case was discovered at a quail farm in the same province about 170 km (100 miles) south of Seoul, some 18 km from the original outbreak, according to the agriculture ministry.

The farm has 290,000 quail and about three thousand had died over the past four days.

Quarantine authorities would cull 360,000 poultry within a 500 metre (1,640 ft) radius of the latest infected farm.

There were no reports to suggest residents or quarantine officials had been infected in or abound the infected farms, the official said.

The infected three farms lie on a path for migratory birds that head south from Russia, Mongolia and Kazakhstan and sparked concern that other parts of South Korea could become infected.

Between December 2003 and March 2004, about 400,000 poultry at South Korean farms were infected by bird flu.

During that outbreak, the country destroyed 5.3 million birds and subsequent testing in the United States indicated at least nine South Korean workers involved in the culling had been infected with the H5N1 virus. None developed major illnesses.

Bird flu remains essentially an animal disease, but it has infected nearly 260 people worldwide since late 2003, killing more than 150, according to the World Health Organisation.

Since 2003, outbreaks have been confirmed in about 50 countries and territories.