South Korean prosecutors on Friday demanded a prison term for a former professor at Yonsei University, who had said in a lecture that "comfort women" -- the euphemism for the tens of thousands of women coerced into sexual slavery by imperial Japan during World War II -- became prostitutes voluntarily.

Ryu Seok-chun, who formerly taught sociology, sparked controversy in September 2019 after he told his class that the comfort women system was "a sort of prostitution." The prosecution is pressing defamation charges, seeking a jail term of one year and six months.

"The accused lectured falsehoods about the comfort women victims without conducting studies or verification of the facts related to the system, thereby infringing great psychological pain upon the victims," the prosecution said. "Since the defendant's remarks and submitted materials show mere suspicions rather than confirmed facts, his remarks constitute defamation."

Last month, South Korean victims of Japan's wartime sexual slavery won a damages suit against the Japanese government when Seoul High Court demanded that Japan pay compensation to the victims. Japan did not appeal the case, confirming the ruling.

Ryu had claimed in a 2019 lecture that Justice for the Comfort Women, a rights organization fighting for legal compensation for comfort women, had educated the victims so that they would state they had been forced into sexual slavery by Japan. He also claimed that the organization had been made up of members of the now-defunct minor opposition Unified Progressive Party, and that they had been linked and loyal to North Korea.

In the same lecture, Ryu was also accused of sexual harassment when he said that the process of getting into prostitution is partially voluntary, and asked a student "if she'd like to try." This led the university to suspend him from his duties.

Ryu said that even though he did not write a paper on the issue of the comfort women, he did in fact have knowledge on the matter.

Ryu's trial is set for Jan. 10, 2024.