Korean students at Harvard University have strongly criticized a professor over his controversial claim that Japan's wartime sexual slavery was actually voluntary prostitution, demanding its immediate withdrawal and his official apology to victims.

Harvard Korean Society made the demand in a statement on its website after Harvard Law School Japanese legal studies professor J. Mark Ramseyer caused controversy with his recently published paper titled "Contracting for Sex in the Pacific War."

"It is a wrong conclusion based on grounds very biased and lacking trustworthiness," the statement said. "Harvard Korean Society demands Prof. Ramseyer's official apology and immediate withdrawal of the paper."

"The issue of comfort women is an international inhumane act, and his academic view which justifies and negates the act is an immoral and shameless view," it added.

The society also demanded the paper be taken down from the International Review of Law and Economics, where it was published, saying that his research has caused pain to victims not just in Korea but also in many other countries.

The criticism marked the latest after a group of Korean students at Harvard Law School last week decried the professor's claim as "factually incorrect and misleading."

Historians have said around 200,000 Asian women, mostly Koreans, were forcibly sent to front-line brothels to provide sex services for Japanese soldiers during World War II.

The issue of the victims, euphemistically called comfort women, resurfaced after a Seoul court ordered Japan last month to make reparations of 100 million won ($88,944) each to 12 former sex slaves -- without recognizing sovereign immunity in the trial.

Tokyo maintains the comfort women issue was permanently resolved through a bilateral agreement in 2015 with the South Korean government. But the victims have called the agreement inadequate, saying it lacks a sincere apology from Tokyo and left out their voices in the negotiation process. (Yonhap)