The record was discovered at the U.S. National Archives and Records Administration in College Park, Maryland, during the research team's monthlong field study from mid-July to August.

Words such as "whores," "comfort women" and "prostitutes" were used throughout other relevant records, indicating the 30 women mentioned in the page were former sex slaves, said professor Kang Sung-hyun, a member of the research team.

The existence of this record was already revealed to the public in the 1990s, but the latest finding was the first time the exact institution holding the document has been identified, said the professor at the Institute for East Asian Studies under Sungkonghoe University in Seoul.

"Our team has been delving into finding official documents on the comfort women issue since 2013 and we finally discovered over a dozen documents this year," he said. "The outstanding feature of this operation diary, in particular, is that it is written in active voice, clearly stating who killed (the victims). It was the Japanese military."

The finding was first presented to the public during an academic workshop held at SNU in southern Seoul on Friday.

"We discovered a total of 113 documents, including 14 on the Japanese military's massacre of sex slaves, this year," the professor told Yonhap News Agency. "We are planning to issue a source book next year based on what we have found so far."

Japan has paid 1 billion yen (US$9.69 million) to a South Korean foundation aimed at helping the victims under a deal reached between the two countries in December. Many South Korean victims, however, still oppose the agreement, saying it lacks Japan's acknowledgment of any legal responsibility.

Historians estimate that up to 200,000 women, mostly from Korea, were forced to work in front-line brothels for Japanese troops during World War II. The issue is increasingly urgent, as most of the surviving South Korean victims are in their late 80s. (Yonhap)