South Korea's health minister apologized on Sunday for failing to halt an outbreak of the MERS virus, vowing the "utmost efforts" to halt the disease's spread as the number infected rose to 15.

"We apologize for causing concern and anxiety among people due to ... our initial judgment on the contagiousness of MERS," Minister Moon Hyung-pyo said.

Moon added that this week would be a "critical period" to contain the spread of MERS, or Middle East respiratory syndrome, which can cause symptoms ranging from flulike aches and pains to pneumonia and kidney failure.