SAN FRANCISCO — The mayor of Osaka, Japan, says he's ending a six-decade "sister city" relationship with San Francisco to protest a statue honoring women forced to have sex with Japanese soldiers during World War II.

Osaka Mayor Hirofumi Yoshimura this week sent a letter to San Francisco announcing he's withdrawing from the largely ceremonial relationship, the San Francisco Examiner reported Wednesday.

The statue was erected on city property last year by California's Korean, Chinese and Filipino communities.

Jeff Cretan, a spokesman for San Francisco Mayor London Breed, called Yoshimura's decision "unfortunate" and says the cities will remain connected through "people-to-people ties."

The Japanese consulate in San Francisco declined to comment.

"Breaking the relationship over a memorial is outrageous and absurd," said Lillian Sing, co-chair of the Comfort Women Justice Coalition. "It shows how afraid the Osaka mayor and Japanese prime minister are of truth and are trying to deny history."

Historians say tens of thousands of women around Asia were sent to work in Japanese military brothels, often through coercion and deception. Japan apologized in 1993 but the issue has remained an open rift with its neighbors, particularly South Korea which has strong memories of Japan's colonization from 1910 to 1945.

After a gradual pullback from the apology, Japan's government now denies that the women, called "comfort women" in Japanese, were forced into sexual slavery, citing a lack of official documentary proof, and says the statue in San Francisco and similar statues built in various countries wrongfully blame Japan.

Follow NBC Asian America on Facebook, Twitter, Instagram and Tumblr.