SEOUL, South Korea (AP) — South Korean trainee doctors collectively walked off their jobs Tuesday to escalate their protest of a government medical policy, triggering cancellations of surgeries and other medical treatments at hospitals.

The Health Ministry urged them to return to work immediately, saying they must not endanger the lives of patients to fight the government.

As of Monday night, more than half of the 13,000 trainee doctors in South Korea were confirmed to have submitted resignations en masse to protest a government push to increase the number of medical students. A total of 1,630 of them have left their work sites, according to Health Ministry records.

More trainee doctors are expected to follow suit. Under a decision made by their association last week, trainee doctors at the country’s five major hospitals were supposed to walk off collectively on Tuesday.

At the center of the dispute is a recent government announcement that it would raise medical school admissions by 2,000 from next year. The government says it's urgent to have more doctors to address what it calls a shortage of doctors, given the country's fast-aging population.

But the plan triggered a strong backlash from many doctors, who say the 2,000 new admissions are too many to be handled by medical schools and that resources must be used to resolve other issues first. They say producing too many doctors would also lead to unnecessary medical treatments due to increased competition among doctors. But their critics argue that doctors mainly worry their incomes would drop if there were more doctors.

Most of the 13,000 trainee doctors work at 100 hospitals across South Korea, assisting senior doctors during surgeries and treating patients. If their walkouts are prolonged or joined by senior doctors, that could cause disruptions at those hospitals and in South Korea’s overall medical service, observers say.

South Korea has a total of 140,000 doctors. The Korea Medical Association said Monday it plans to hold rallies to support trainee doctors’ collective action but hasn't determined whether to launch strikes.

At Seoul’s Asan Medical Center, a nurse said it’s unclear how long senior doctors could solely perform surgeries and other treatments without the assistance of trainee doctors. The nurse, who requested anonymity citing the sensitivity of the issue, said that trainee doctors typically handle skin incision and disinfection during surgeries at the direction of senior doctors and manage data on hospital computers.

She said her hospital plans to delay the planned acceptance of some cancer patients and to release inpatients early. Other Asan hospital officials said Tuesday that an unspecified number of trainee doctors didn’t come to work but noted some are still working. They said the hospital is rearranging planned surgery schedules by reviewing the conditions of patients.

Vice Health Minister Park Min-soo said authorities have received 34 public complaints related to the trainee doctors' walkouts – 25 of them over the cancellation of surgeries. Other cases include hospitals refusing to treat patients and canceling planned medical treatments.

“If you leave your patients to oppose a government policy despite knowing what your collective action would cause, that can't be justified at all," Park said. “We appeal to trainee doctors to return to patients. An act of endangering the lives of patients to express your opinions is something that you must not do.”