SEOUL - South Korea's government late on Friday publicly ordered 13 doctors, at least some of whom have been vocal about a walkout by roughly 9,000 physicians over a health system reform plan, to return to work or face penalties.

The public legal notice is the latest sign of the South Korean government's continued hardline stance toward trainee doctors participating in the walkout or criticising the reform plan, after local police raided officials of a doctor's association on Friday, a public holiday in South Korea.

The health ministry posted on its website the license numbers and hospitals of 13 doctors, ordering them to return to work or potentially have their license suspended or face criminal charges.

The doctors' names were partially redacted, but at least some appeared to be trainee doctors who had been especially vocal about the walkout and critical of the government, including Park Dan, head of the Korea Interns and Residents Association.

Authorities on Friday stepped up pressure to end the walkout, with South Korean police launching a raid targeting officials of the Korean Medical Association.

The government had given Thursday as the deadline for the doctors to return or face penalties, but health ministry data showed more than two-thirds of the trainee doctors, or nearly 9,000, had ignored the call to return to work.

Doctors are planning a mass demonstration on Sunday to protest the government's plan to increase medical school admissions by 2,000 starting from 2025 to remedy what it said is a shortage of doctors in one of the world's fastest-ageing societies.

(Reporting by Joyce Lee Editing by Chris Reese)