The new bird flu outbreaks were discovered at two duck farms in the central and southwestern regions of the country, the Ministry of Agriculture, Food and Rural Affairs said in a statement on Wednesday. All 30,500 ducks at the farms have been culled.

One more case was confirmed on Tuesday and the second earlier on Wednesday, the ministry statement said.

The third case was confirmed later on Wednesday at a chicken farm in the city of Yangju, about 38 kilometers (23.61 miles)north of Seoul, the ministry said.

Since the first cases of the H5N6 avian influenza virus were reported in the same region on Friday, a total of five cases have been found and two other poultry farms in other parts of the country are being tested, the ministry said.

The agriculture ministry raised its bird flu alert level to 'alert,' from 'caution,' as an increasing number of farms are reporting suspected infections in a short period of time.

Over 510,000 birds have been slaughtered to contain the spread of the virus, accounting for less than 1 percent of the country's poultry population of 84.7 million, an agriculture ministry official who declined to be identified said on Wednesday.

Cases of human infection from the H5N6 virus have previously been reported in places including China and Hong Kong, with the virus killing 10 people in China since April 2014, according to data from the ministry.

The agriculture ministry has already put in place a poultry movement ban for farms within a 10-kilometre radius of where the virus had been found earlier.

The ministry said it is also considering a temporary nationwide standstill order to prevent the virus from spreading.

There have been previous bird flu cases in South Korea, Asia's fourth-largest economy, since 2014.

The outbreaks lasted for more than 660 days, killing at least 13 million birds, ministry data showed. The most recent bird flu outbreak was reported in March, when the H5N8 strain was found at a duck farm.