After boycotting one organised by Tokyo, South Korea held its own memorial event on Monday for compatriots forced to work in a Japanese mine during World War Two, highlighting lingering sensitivities between the countries.

Both key U.S. regional allies, Seoul and Tokyo have seen ties generally improve in recent years, as they sought to put aside acrimonious disputes tied to Japan's 1910-45 colonisation of the Korean peninsula.

Seoul's decision not to attend Sunday's official event at the Sado mine followed reports that Japan's government would be represented by an official who had visited a shrine neighbouring countries see as a symbol of Japan's militarist past.

The governments could not resolve their differences in time, South Korean Foreign Minister Cho Tae-yul said in a weekend television interview, but added the incident should not damage improved ties under President Yoon Suk Yeol.

Yoon has pushed to boost three-way security efforts with Tokyo and Washington.

South Korea's ambassador to Japan led the memorial event, with the participation of nine descendents of the original labourers as well as some government officials.

In his remarks, Ambassador Park Cheol-hee expressed deep sorrow at the plight of South Korea's forced labourers, exhorting Japan to ensure the painful history of the mine was not forgotten.

"Our government's decision not to attend the memorial service by the Japanese side and hold its own memorial event is an expression of our government's firm determination not to compromise with the Japanese side on the past history," South Korea's foreign ministry said in a statement.

Japan's top government spokesman, Yoshimasa Hayashi, said it was not Tokyo's place to explain South Korea's absence from the memorial ceremony held by Japan's government.

"But we do think it is regrettable that they did not participate," he told a daily briefing in the Japanese capital.

Hayashi added that Tokyo had explained to Seoul that Akiko Ikuina, the official who represented Japan at the ceremony, had not visited the controversial Yasukuni shrine since becoming a lawmaker.

South Korea negotiated a pact with Japan to hold events explicitly mentioning South Korean forced labourers as a condition for backing Tokyo's bid to list the Sado mine as a UNESCO World Heritage Site.

But Yoon's government drew criticism at home for not pushing Japan harder to recognise the darker aspects of the mine, which Japan celebrates for its long history and contribution to the country's industrial development.