Japan held a memorial ceremony on Sunday near the Sado Island Gold Mines despite a last-minute boycott of the event by South Korea that highlighted tensions between the neighbors over the issue of Korean forced laborers at the site before and during World War II. South Korea's absence at Sunday's memorial, to which Seoul government officials and Korean victims' families were invited, is a major setback in the rapidly improving ties between the two countries, which since last year have set aside their historical disputes to prioritize U.S.-led security cooperation. The Sado mines were listed in July as a UNESCO World Heritage site after Japan moved past years of disputes with South Korea and reluctantly acknowledged the mines' dark history, promising to hold an annual memorial service for all victims, including hundreds of Koreans who were mobilized to work in the mines. On Saturday, South Korea announced it would not attend the event, saying it was impossible to settle unspecified .