TOKYO (Reuters) - The mayor of Japan’s western city of Osaka plans to cut ties with U.S. sister city San Francisco after the latter accepted the donation of a “comfort women” statue from a private group there.

The issue of “comfort women”, as those forced to work in Japan’s wartime military brothels were euphemistically known, has long embittered the ties of neighbors, such as China and South Korea, with Japan.

“This is highly regrettable,” Osaka Mayor Hirofumi Yoshimura told reporters, describing Wednesday’s endorsement by San Francisco Mayor Edwin Lee of a city council decision. “The relationship of trust has completely been destroyed.”

Yoshimura aims to complete the procedures necessary to snap ties by the end of the year, he said in a statement on Thursday.

No officials at the San Francisco mayor’s office were immediately available for comment.

“Erecting comfort women statues in the United States and other countries is in conflict with our country’s stance and extremely regrettable,” Japanese Chief Cabinet Secretary Yoshihide Suga told a regular news conference on Friday.

“We plan to continue making every effort so that things like this won’t happen again.”

In January, Japan temporarily recalled its ambassador to South Korea over a “comfort women” statue put up near its consulate in the southern city of Busan.

In 2015, Japan and South Korea agreed the issue of “comfort women” would be “irreversibly resolved” if both sides fulfilled their obligations, including a Japanese apology and a fund to help victims.

But South Korean President Moon Jae-in has said many South Koreans did not accept the deal reached by his conservative predecessor and Japanese Prime Minister Shinzo Abe.

On Friday, Suga said Japan had protested to Seoul after the South Korean parliament passed a bill designating Aug. 14 as a day of commemoration for “comfort women”, adding that the move risked affecting ties, Kyodo news agency reported.