SEOUL, April 17 (Reuters) - South Korea said on Thursday it had culled three million farmed birds and was probing seven fresh cases of suspected bird flu, as the country grapples with its worst avian influenza outbreak in four years.

In just two weeks, South Korea has confirmed 12 cases of the deadly H5N1 strain, raising alarm as the highly virulent virus is spreading at its fastest rate since the country reported its first case in 2003.

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

The country raised the risk level for bird flu to the second highest on Wednesday and sent 200 soldiers to kill and bury birds, as an outbreak was confirmed at a farm in Pyeongtaek, just 60 km south of Seoul, bringing the disease closer to the capital.

The ministry said on Thursday its quarantine work would focus on speeding up slaughtering and investigating possible causes of the spread such as migrating birds and transport workers who have been moving around affected sites and other parts of the country.

South Korea had seven bird flu outbreaks between November 2006 and March last year and 19 cases between December 2003 and March 2004, when it had to kill 5.3 million birds.

There have been 240 human deaths globally from the H5N1 strain and 380 confirmed cases of infection since 2003, according to World Health Organisation data. (Reporting by Miyoung Kim; Editing by Keiron Henderson)

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