SADO, Japan (AP) — South Korea paid tribute to wartime Korean forced laborers at Japan’s Sado Island Gold Mines in a memorial ceremony on Monday, a day after boycotting a similar event organized by Japan, as tensions over historical atrocities continue to impact relations between the two sides.

Japan has held a memorial ceremony 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 brutal wartime use

The event came after the Sado Island Gold Mines were designated a UNESCO World Heritage site earlier this year.

The Japanese government on Monday expressed regret over South Korea boycotting a Japan-led event honoring wartime forced labor victims, including Koreans, from an old Japanese mine complex. The remark came as South Korea boycotted the memorial held on Sado Island,

South Korea held a memorial event in Japan to honor laborers forced to work during WWII, boycotting a Japanese-organized event due to sensitivities over historical disputes. Improved ties between Seoul and Tokyo face challenges as both nations navigate diplomatic relations tied to colonial history.

North Korea condemned on Saturday recent joint military drills by the United States, South Korea and Japan, warning that it would take immediate actions if needed to defend the state.

Efforts by South Korea and Japan to hold a joint memorial service near Japa’s Sado Island Gold Mines, where Koreans were forced to work during Japan’s colonial rule, fell apart on Nov. 24 due to disagreements between both governments.