South Korean President Yoon Suk Yeol on Monday slammed the "cartel" of medics who oppose sector reforms, saying the government would not back down on plans that have triggered a month-long doctors' strike.

Hospitals have been forced to cancel crucial treatments and surgeries since thousands of trainee doctors stopped working February 20 to protest against proposed training reforms, but the government has not changed tack, instead threatening striking medics with legal action.

Seoul wants to increase medical school admissions by 2,000 starting next year, saying it is essential to combat shortages and treat a fast-ageing society. Doctors say it will erode service quality.

"The number 2,000 is not a random figure we came up with. We have thoroughly reviewed relevant statistics and research and reviewed present and future medical situations," Yoon said in a televised national address.

Even this increase will not meet growing demand in areas outside the capital Seoul, he said.

Thousands of doctors are facing the suspension of their medical licences, but Yoon urged medics to return to their hospitals before the process was complete.

Successive South Korean governments have tried -- and failed -- to increase medical school admissions in the past, and Yoon said "the cartel of doctors had been strengthened" by every previous failure.

"We cannot repeat the same mistake again," he added.

If the doctors' community don't like the government's plan, "they should present the government a unified blueprint with clear scientific reasoning," he said.

"If they bring an alternative that is more rational and reasonable, we can talk anytime."

South Koreans will vote next week in a crucial election, with Yoon's party trying to win back its majority in parliament.

The public initially sympathised with the government in the ongoing doctors' strike, but recent polls have indicated sentiment has shifted, with nearly 60 percent of people surveyed in a Dong-A Ilbo poll Monday saying the government should adjust the scale and timing of the reform.

The opposition Democratic Party slammed Yoon for being "preoccupied with the number 2,000" and urged him to adjust the reform plan "taking into account medical situations".

"Yoon and the government must abandon their obsession with the 2,000-slot increase," said opposition Democratic Party MP Shin Hyun-young.