SEOUL, South Korea (AP) — South Korean officials issued return-to-work orders for doctors participating in a one-day walkout Tuesday as part of a protracted strike against the government’s plan to sharply boost medical school admissions.

Since February, more than 12,000 trainee doctors have been on strike in a deepening standoff with government officials, who want to grow the country’s number of doctors by up to 10,000 by 2035. Many reject the plan, set to begin next year, saying schools won’t be able to handle the increased flow and that the quality of the country’s medical services would suffer.

About 4% of the country’s 36,000 private medical facilities, categorized as clinics, have told authorities they would participate in a one-day strike on Tuesday, according to South Korea’s Health and Welfare Ministry.

This came a day after hundreds of medical school professors at four major hospitals affiliated with Seoul National University entered an indefinite walkout, raising concerns about disruptions in medical services.

There’s also a possibility that the strike could expand.

At a rally with thousands of doctors in Seoul on Tuesday, Lim Hyun-taek, the hard-line leader of the Korean Medical Association, said he would push for its members to enter an indefinite strike on June 27 if the government rejects its demands to completely scrap plans to increase medical school admissions. KMA is the country’s largest doctors’ lobby with more than 100,000 members.

South Korean Deputy Health Minister Jun Byung-wang said the one-day strike by clinics and the walkout by SNU-affiliated medical professors haven’t immediately caused significant problems in medical services.

He accused the protracted strike of threatening to destroy a “trusting relationship between doctors and patients our society has built for long.”

“We cannot allow unlimited freedom to the medical profession,” Jun said Tuesday. “Since they benefit from a medical licensing system that limits the supply (of doctors) and ensures their monopoly of the profession, doctors must uphold their end of professional and ethical responsibilities and legal obligations under the medical law.”

Under South Korean law, doctors defying return-to-work orders can face suspensions of their licenses or other punishment.

Jun said they planned to request hospitals to pursue damage suits against the striking medical professors if their walkouts prolong and disrupt medical services. He said hospitals that fail to sufficiently respond to the walkouts may face disadvantages in health insurance compensation and that the government plans to push legal action against any hospital that cancels reserved treatments with patients without notifying them in advance.

In a cabinet meeting on Tuesday, President Yoon Suk Yeol called the monthslong strike “regrettable” and warned that his government will sternly respond to “illegal activities that abandon patients.”

The striking doctors suffered a significant legal setback in May when the Seoul High Court rejected their request to block the government plan, which would raise the yearly medical school enrollment quota by 2,000 from the current cap of 3,058.

South Korea’s doctor-to-population ratio is one of the lowest in the developed world.

Government officials say the country needs significantly more doctors to cope with the fast-aging population and have downplayed doctors’ concerns about a possible decline in future incomes.

The striking doctors are a fraction of all doctors in South Korea, estimated to number between 115,000 and 140,000. Still, the walkouts have resulted in cancellations of numerous surgeries and other treatments at some large hospitals, which are more dependent on junior doctors and trainees.

Government officials earlier threatened to suspend the licenses of the striking doctors but later halted those administrative steps to facilitate dialogue.