SEOUL, Nov 27 (Reuters) - South Korea expects to cull 236,000 poultry by the end of the week to prevent spreading of the highly virulent H5N1 strain of bird flu, after the country´s first outbreak in three years, officials said on Monday.

The Agriculture Ministry will cull all poultry within a 500-metre (1,640-ft) radius of a farm in North Cholla province about 170 km (100 miles) from Seoul where officials said on the weekend the avian influenza strain had been detected.

The quarantine will also include culling about 300 pigs and 600 dogs within the area.

"We finished culling of all poultry at the infected farm on Saturday and began slaughtering other poultry near the farm from yesterday," said an official at the Agriculture ministry.

Quarantine authorities also banned the shipment of more than 5 million poultry from 221 farms within a 10-km (6.2-mile) radius of the farm.

Shares in the country´s top chicken meat processor Halim Co. Ltd. were down 3.07 percent at 2,525 won by 0302 GMT, underperforming a 0.3 percent fall in broader market.

North Korea is also stepping up measures to prevent bird flu, its official media said on Monday.

North Korea, which had an outbreak of bird flu at two poultry farms near Pyongyang in February 2005, said it had inoculated poultry and increased checks along its borders.

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

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