South Korea raised its health alert to the highest level on Friday after a mass walkout by trainee doctors this week, while the prime minister said public hospitals would extend working hours to respond to growing strains on the medical system.

Almost two-thirds of the country's young doctors have walked off the job to protest a government plan to admit more students to medical schools, forcing hospitals to turn away patients and cancel procedures, and raising fears about further disruption to the medical system should the dispute drag on.

"The operation of public medical institutions will be raised to the maximum level," Prime Minister Han Duck-soo said at the opening of a disaster management meeting.