South Korea has discovered a fifth case of bird flu despite the culling of poultry after earlier cases, a government official said on Saturday, raising concerns that quarantine measures had failed to control the outbreak.

The latest case occurred at a poultry farm in Chonan, South Chungcheong province, around 90 km (55 miles) south of the capital, Seoul, and about 8 km from Asan where the fourth case was found.

"We confirmed that a case at the farm was highly pathogenic," an agriculture ministry official said by telephone.

Quarantine authorities would cull poultry within a 500-meter radius of the infected farm, the official added.

There were no reports suggesting human infection.

In November South Korea confirmed its first case of the H5N1 strain in about three years.

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 Organization.

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