SEOUL, April 15 (Reuters) - South Korea said on Tuesday it was investigating four new suspected cases of bird flu, including one near Seoul, adding to a string of recent outbreaks and raising concerns the disease may be spreading across the country.

South Korea has confirmed 11 cases of bird flu in less than two weeks, but all have been contained to the southwest of the country -- North and South Jeolla provinces, some 320 km (200 miles) south of Seoul.

"We have received four fresh reports of suspected bird flu cases on Monday and one is from a poultry farm in Pyongtaek city in Kyonggi province," the Farm Ministry said in a statement.

Pyongtaek is just 60 km south of Seoul.

South Korea, which has culled more than half a million poultry this month, said it would continue quarantine work in suspected areas and encourage poultry consumption amid rising consumer concern.

Duck and chicken prices have dropped 10 percent this month, while chicken sales at four major retailers surveyed by the ministry showed a 20 percent decline.

South Korea had seven outbreaks of the deadly H5N1 strain between November 2006 and March 2007 and spent 58 billion won ($59.22 million) on quarantine measures.

There had been 238 human deaths globally from the H5N1 strain and 376 confirmed cases of infection since 2003, according to World Health Organisation data.

A major concern is the possibility of mutation into a disease that easily passes from one person to another, triggering a global pandemic. (Reporting by Miyoung Kim; Editing by Nick Macfie)

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