The International Labour Organization has said it will not intervene in a South Korean doctors strike that has plunged the country's hospitals into chaos.

Thousands of trainee doctors stopped working last month to protest a government plan to ease doctor shortages by increasing the number of medical students, a move they say will erode the quality of education.

Seoul has demanded they return to work, threatening legal action and suspending the licenses of two doctors for "instigating" the work stoppages, which are tightly restricted under South Korean law.

Last week, a group of junior doctors said they had appealed to the ILO for assistance, claiming they were being "forced" to work on pain of legal action due to the government's draconian response.

The ILO told AFP late Wednesday it was "aware of the unrest in the Republic of Korea resulting from a complex dispute between health care workers and the government and involving allegations of forced labour."

However, it said it had "not been formally approached by its constituents" for intervention, urging both sides to engage in dialogue to refrain from "violence and coercive measures" in the standoff.

Seoul's Labour Ministry on Thursday said the Korean Intern Resident Association, the body that asked for help, "did not meet the criteria" of an ILO constituent, which is limited to government, domestic or international labour unions.

The government has previously said the ILO's ban on forced labour does not apply in circumstances where "a disaster or crisis endangers the survival or welfare of an entire population or group".

In a statement, the ILO told AFP that "legislation allowing the call-up of workers in cases of emergency should be worded in terms not so broad as to call into question the fundamental principle of the elimination of forced labour".

In particular, it added, it was good to avoid situations in which "persons requisitioned who refuse to work are liable to criminal penalties".

South Korea on Wednesday announced the allocation of 2,000 new medical school admissions slots nationwide every year, moving ahead with a reform plan to create more doctors despite the walkout.

While doctors say the reforms will harm the country's quality of medical service, proponents say they are simply trying to safeguard their salaries and social status.