SEOUL (Reuters) - South Korea´s health ministry said on Thursday a poultry worker was infected with the H5N1 strain of bird flu late last year, but had not been seriously ill.

The person was infected after an outbreak of the deadly strain that first hit poultry farms in the country in November.

"We confirmed the person was positive for antibodies to bird flu," the ministry said in a statement.

The poultry worker was subject to regular testing of farmers and workers involved in a mass cull of about 1.7 million fowl following the discovery of the H5N1 virus at four farms in and around Iksan, about 170 km (100 miles) south of Seoul.

The worker did not develop any serious illness and is now healthy.

The four farms lie on a path for migratory birds that head south from Russia, Mongolia and Kazakhstan.

Tests have been completed on 26 of 85 farmers and those carrying out the cull in Iksan. Results on the remaining people should be finished by around the end of this month, the ministry said.

During an H5N1 outbreak in late 2003 and early 2004, several South Koreans were infected but none developed any major illnesses, South Korean health officials said.

The World Health Organization said from 2003 and as of January 10, there have been 264 confirmed human cases of H5N1 infection, or which 157 people have died. Vietnam, with 42 deaths and Indonesia with 57 deaths, have been the two countries hardest hit.