South Korea reported four new cases of MERS on Wednesday, its third consecutive day without any fatalities, as the total number of infections rose to 179.

The four new cases of Middle East Respiratory Syndrome included two who were in the same hospital ward as other MERS patients, the health ministry said.

Two others were a nurse at Samsung Medical Centre in Seoul -- one of the epicentres of the outbreak -- and a relative of a patient who was hospitalised for an unspecified disease in a hospital in Pyeongtaek, south of Seoul, in early June.

Authorities were still investigating how exactly the latter two had contracted the virus, the ministry said.

The total number of people who had died in the outbreak remained unchanged at 27, it said, adding that about 3,100 people were still under quarantine at state facilities or at home.

Sixteen patients were in critical condition, it said, while a total of 67 people had recovered and were released from hospital, including 13 from Monday to Tuesday.

The outbreak -- the largest outside Saudi Arabia -- began on May 20 when a 68-year-old man was diagnosed after returning from a trip to Saudi Arabia.

The virus since then has spread at an unusually rapid pace, sparking alarm in the Asia's fourth-largest economy and elsewhere in Asia.