A new book offering detailed historical facts about "comfort stations" during World War II has been released at the ongoing Shanghai Book Fair.

"Comfort station" is a euphemism the Japanese occupation forces used to describe a military brothel.

Based on historical documents and accounts of witnesses and victims, the book provides readers with a detailed look at the 172 comfort stations in Shanghai during the WWII.

"Over the past 25 years, we discovered the evidence that there were at least 172 comfort stations in Shanghai, the most of any city worldwide," said Su Zhiliang, the book's author and a leading expert on "comfort women," girls and women forced into sex slavery by the Japanese during WWII.

Some 400,000 women across Asia were forced to become comfort women for the Japanese army, and nearly half of them were Chinese, according to the Research Center for Comfort Women at Shanghai Normal University.

"I have been researching the issue for 25 years and will continue to ask for justice for the 400,000 Asian victims," said Su, who is the director of the center.

There are only 15 known surviving comfort women on the Chinese mainland. They have an average age of over 90.