South Korean health authorities said Thursday that they are currently testing four suspected cases of swine flu amid escalating fears the deadly virus could evolve into a global pandemic, according to Yonhap News Agency Thursday.

The Korea Centers for Disease Control and Prevention (KCDC) said in a statement that it is investigating four individuals after they were reported to have been showing flu-like symptoms such as coughing and fever.

The four are among a total of 23 people who have contacted authorities over possible infections. Eighteen people tested negative but a 51-year-old woman has been quarantined for treatment after she was classified as the nation's first "probable" case of swine flu Tuesday.

Details on the individuals being tested were not available, including information on when and to where they traveled.

The report comes as governments worldwide are struggling to control the spread of the highly infectious virus, which has reportedly killed over 150 people in Mexico alone.

The first death outside Mexico was confirmed Wednesday in the United States, raising concerns that the virus is spreading at a faster-than-expected pace. Cases have also been reported in other nations, including Canada, Spain and New Zealand.

The World Health Organization earlier warned that swine flue is threatening to become a pandemic, raising its alert level one notch higher.

South Korea has also intensified precautionary efforts by restricting travel to Mexico, the epicenter of the outbreak, while tightening quarantines of pork imported from affected countries.

On Tuesday, the disease control center raised its alert level by one notch to "Yellow" from the previous "Blue," a move aimed at intensifying cooperation to stem the outbreak of swine influenza.