Japanese officials on Sunday paid tribute to workers at the country’s Sado Island Gold Mines but offered no apology for Japan’s brutal wartime use of Korean forced labourers, highlighting lingering tensions between the neighbours over the issue.

South Korea a day earlier announced that it was boycotting the memorial, saying it had been impossible to settle unspecified disagreements between both governments in time for the event. The Korean absence is a major setback in the rapidly improving ties between the two countries, which since last year have set aside their historical disputes to prioritise US-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 mobilised to work in the mines.

The first ceremony of what Japan promised to make an annual event took place at a facility near the mines, with more than 20 seats for South Korean attendees remained empty.

“As a local resident, I must say (their absence) is very disappointing after all the preparations we made,” said Sado mayor Ryugo Watanabe. “I wish we could have held the memorial with South Korean attendees.”

Families of Korean victims of mine accidents were expected to separately hold their own ceremony near the mine on Monday.

At Sunday’s ceremony four Japanese representatives, including central and local government officials and the head of the organising group, thanked all mine workers for their sacrifice and mourned for those who died. None offered any apology to Korean forced labourers for the harsh treatment at the mines.

Akiko Ikuina, a parliamentary vice minister, representing Japan’s government, praised the craftsmanship of the labourers and their contribution to the Sado mines.

She noted that “many people from the Korean Peninsula were at the mines under Japan’s wartime labour policies” and that they engaged in difficult work under dangerous and severe conditions away from home and their loved ones, and some died in accidents or from illnesses. But she did not acknowledge their forced labour or Japan’s colonisation of the Korean Peninsula.

There has been speculation that the South Korean boycott might have been because of Ikuina’s past visit to Tokyo’s controversial Yasukuni Shrine — in August 2022, weeks after she was elected as a lawmaker. Japan’s neighbours view Yasukuni, which commemorates 2.5 million war dead including war criminals, as a symbol of Japan’s past militarism.