South Korea's deadly Middle East Respiratory Syndrome (MERS) outbreak is "large and complex" and more cases should be expected, World Health Organization (WHO) experts said on Saturday.
Experts who have reviewed the MERS outbreak with South Korean health authorities have also said there is no sign the transmission of the virus is speeding up.
The outbreak in South Korea, the largest outside Saudi Arabia, has infected 138 people, killing 14 of them since May 20.
"Because the outbreak has been large and is complex, more cases should be anticipated," WHO Assistant Director General Keiji Fukuda told a news conference.
Fukuda also said some social customs, such as large family visits to hospitals, may have contributed to the spread of the infection.
South Korean officials announced 12 new cases on Saturday, including the infection of an ambulance driver who transported a patient, and widened the quarantine. A 14th death from the disease was also reported.
"The government should remain vigilant and should continue its intensified disease surveillance and prevention measures until the outbreak is clearly over," Fukuda said, adding a "great deal of anxiety" persists among Koreans.
The disease was first identified in humans in 2012 in Saudi Arabia and is similar to the Severe Acute Respiratory Syndrome (SARS) which left more than 800 people dead and infected about 8,000 people worldwide between 2002 and 2003.
MERS does not spread from human to human easily but its fatality rate is much higher than SARS, according to WHO figures. The MERS virus has so far caused the deaths of more than 446 people.