SEOUL (Reuters) - South Korea said on Thursday it hadculled three million farmed birds and was probing seven freshcases of suspected bird flu, as the country grapples with itsworst avian influenza outbreak in four years.

In just two weeks, South Korea has confirmed 12 cases ofthe deadly H5N1 strain, raising alarm as the highly virulentvirus is spreading at its fastest rate since the countryreported its first case in 2003.

The farm ministry said on Thursday it had seven new reportsof suspected bird flu outbreaks at poultry farms in North andSouth Jeolla provinces, some 320 km (200 miles) south of Seoul,where the first bird flu recurrence for a year was reportedearlier this month.

The country raised the risk level for bird flu to thesecond highest on Wednesday and sent 200 soldiers to kill andbury birds, as an outbreak was confirmed at a farm inPyeongtaek, just 60 km south of Seoul, bringing the diseasecloser to the capital.

The ministry said on Thursday its quarantine work wouldfocus on speeding up slaughtering and investigating possiblecauses of the spread such as migrating birds and transportworkers who have been moving around affected sites and otherparts of the country.

South Korea had seven bird flu outbreaks between November2006 and March last year and 19 cases between December 2003 andMarch 2004, when it had to kill 5.3 million birds.

There have been 240 human deaths globally from the H5N1strain and 380 confirmed cases of infection since 2003,according to World Health Organization data.

(Reporting by Miyoung Kim; Editing by Keiron Henderson)