KWACHON, South Korea, Nov 23 (Reuters) - South Korea´s farm ministry said on Thursday it had discovered a suspected case of bird flu at a poultry farm in the southwest of the country, which could be the country´s first outbreak in about three years.

The official said about 6,000 chickens at a farm in North Cholla province had died this week. The remaining 6,000 or so poultry at the farm would now be culled.

"This case appears quite likely to involve highly pathogenic avian influenza," Kim Chang-sub, a top official at the ministry´s quarantine department, told a news conference.

"The final results of our testing should be known on Nov. 25," Kim said, adding he would not speculate as to whether it was the H5N1 strain, which is potentially deadly to human beings.

The report comes as migratory birds typically pass through the Korean peninsula, heading south for the winter from places such as Russia, Kazakhstan and Mongolia. The farm being investigated lies on a migration path, the ministry said.

Kim said Seoul would strengthen preventative measures, which include keeping farm poultry indoors during the migration season and checking wild bird flocks for signs of avian influenza.

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

During that outbreak, the country culled 5.3 million birds and spent about 1.5 trillion won ($1.6 billion) to prevent the disease spreading, officials said.

Subsequent testing in the United States indicated that at least nine South Korean workers involved in the mass cull had been infected with the H5N1 virus but none of them developed any major illnesses, South Korean health officials have said.

Since the virus re-emerged in Asia in 2003, outbreaks have been confirmed in around 50 countries and territories, according to data from the World Organisation for Animal Health.

North Korea had an outbreak of bird flu at poultry farms near the capital, Pyongyang, in February 2005, which caused it to cull more than 200,000 chickens and vaccinate 1.1 million poultry.

The World Health Organisation said that as of Nov. 13, there had been 258 cases of human infection of the H5N1 strain since 2003, killing 153 people. Many of the victims were asians, with 98 deaths in Vietnam and Indonesia, it said