South Korea has killed nearly 100,000 chickens and ducks to try to prevent the spread of bird flu after an outbreak of the deadly H5N1 strain at a chicken farm last week, officials said Monday. Quarantine officials have killed 96,000 poultry within a 500-meter (1,650-foot) radius of the outbreak site in Iksan, about 250 kilometers (155 miles) south of Seoul, the Agriculture Ministry said. South Korea plans to cull a total of 236,000 birds and an unspecified number of other animals like pigs, dogs and cats in the area by Thursday, the ministry said. It also plans to destroy some 6 million eggs. Officials determined Saturday that the outbreak was caused by the H5N1 virus and began culling birds Sunday. South Korea killed 5.3 million birds during the last known outbreak of bird flu in 2003. The H5N1 virus began ravaging Asian poultry stocks in late 2003 and has killed at least 153 people worldwide. So far, the disease remains hard for people to catch, and most human cases have been traced to contact with infected birds. But experts fear it will mutate into a form that is easily spread among people, possibly creating a pandemic that could kill millions. South Korea was also hit by a low-grade strain of bird flu. In two separate outbreaks, bird flu killed 200 chickens in Pyeongtaek, 70 kilometers (43.5 miles) south of Seoul, and some 510 chickens in Yangpyeong, 55 kilometers (34.18 miles) east of the capital. The Agriculture Ministry said the viruses found in both cases were not the H5N1 strain, which unlike most bird flu viruses is harmful to humans. North Korea, meanwhile, has stepped up measures to prevent bird flu, by inoculating poultry and closely monitoring migratory birds, the country's official Korean Central News Agency reported Monday. Bird flu hit North Korea early last year, prompting the slaughter of about 210,000 chickens and other poultry. No new cases of bird flu have since been reported.