SEOUL (Reuters) - South Korea said on Thursday it had culled 3 million farmed birds and confirmed three more outbreaks of bird flu, as the country grapples with its worst avian influenza outbreak in four years.

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In just two weeks South Korea has confirmed 15 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.

It also said it would slaughter hundreds of thousands of birds in the most severely affected southwestern part of the country, as it confirmed three new outbreaks of the deadly H5N1 in the area late on Thursday.

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.

But South Korean authorities have come under heavy criticism as the highly pathogenic virus continues to spread, despite their rigorous efforts, and as they have failed to provide clear explanations for the cause of the fast spread.

The farm ministry said 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.

The fast-spreading outbreak has hit poultry consumption, posing a big threat to farmers already struggling with high animal feed prices.

Han told related ministers to consider helping farmers with tax benefits, financial assistance and budget spending.

South Korea, which estimates it will spend 32 billion won ($32.4 million) on quarantine work in the current outbreak, also plans to introduce insurance products that offer up to 2 billion won compensation if a human infection is reported following poultry consumption.

Chicken sales in the past 15 days have dropped 60 percent at four major retailers surveyed by the farm ministry, while prices of chicken and duck dropped up to 10 percent.

The country 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. No human deaths have been reported.

Some 240 human deaths have been reported globally from the H5N1 strain and 380 confirmed cases of infection since 2003, according to World Health Organisation data.