South Korea will maximize the use of public hospitals to respond to growing strains on the medical system after a mass walkout by trainee doctors, the prime minister said on Friday, as the public health alert was elevated to the highest level.

The protests by almost two-thirds of the country's young doctors have forced hospitals to turn away patients and cancel procedures, 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.

Public hospitals will extend operating hours and will also open on weekends and holidays, he said.

So far, more than 8,400 doctors have joined the walkout, the health ministry said, equivalent to about 64 percent of the entire number of resident and intern doctors in South Korea.

Emergency departments in South Korea's biggest hospitals have been squeezed since trainee doctors began leaving the job this week in protest at government plans to increase medical school admissions to bolster the healthcare sector.

The growing pressure on hospitals has prompted the government to raise its health alert to "severe" as of Friday morning the health ministry said late on Thursday.

The doctors taking part in the protest say the real issue is pay and working conditions, not the number of physicians.