South Korea marked its first “memorial day” on Tuesday for girls and women forced to work in Japan’s wartime brothels, with both of the U.S. allies expressing concern that the emotionally charged issue could undermine their relations.

Under a 2015 deal, Japan apologised to the “comfort women”, its euphemism for women – many of them Korean – forced to work in its wartime brothels, and provided a 1 billion yen (now £7 million) fund to help them.

But South Korea has recently sought to revisit the issue.

“I hope that this issue will not lead to a diplomatic dispute between Korea and Japan. Nor do I see this is an issue that can be solved through diplomatic solutions between the two countries,” South Korean President Moon Jae-in said in comments marking the first “Memorial Day for Japanese Forces’ Comfort Women Victims”.

“It is an issue that can be solved only when the world, including ourselves and Japan, deeply reflects on sexual violence against all women and human rights problems and comes to a strong awareness and learns a lesson in a way that prevents this from ever repeating again.”

Japan insists the issue was resolved by the 2015 deal, struck with a previous, conservative South Korean administration.

A senior Japanese official said differences with South Korea on the matter could undermine efforts to “develop a future-oriented relationship”.

“We are concerned that this may spoil such endeavours,” said the Japanese official, who declined to be identified.

“We have reached out through diplomatic channels to stress the importance of steadily implementing the Japan-South Korea agreement,” the official said, referring to the 2015 deal.

The bitterness over Japan’s wartime actions comes as concern about North Korea’s development of nuclear weapons and missiles has posed a threat to both South Korea and Japan, both of which play host to U.S. military bases.

Moon’s administration has spotlighted the issue and has called for Japan to do more, despite backing down in January from a demand to formally renegotiate the 2015 deal.

In March, Moon described Japan’s wartime use of the women as a crime against humanity. Japanese Chief Cabinet Secretary Yoshihide Suga called the remarks “extremely regrettable”.

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A monument was unveiled as part of the events to mark the day, chosen as it was on Aug. 14, 1991, that South Korean comfort woman victim Kim Hak-sun became the first to give public testimony about her experience.

A think-tank funded by the South Korean government devoted to researching the issue also opened this month.

Japan has protested over monuments in South Korea dedicated to the women, including one in front of the Japanese embassy in Seoul, as well as the decision to designate a day to remember the women.

The issue has been a regular source of animosity between Japan and neighbours including China, North and South Korea and Taiwan, where earlier on Tuesday, more than 50 activists protested in front of Japan’s representative office.

“Japanese government should apologise,” chanted the protesters, many wearing black shirts with their faces covered by white masks. They also demanded compensation for Taiwan women forced to work in the brothels.

A bronze statue symbolising comfort women was also unveiled in the southern Taiwan city of Tainan, the United Daily News reported.

The unveiling was attended by former President Ma Ying-jeou.

Japan colonised the Korean peninsula between 1910 and 1945 and occupied parts of China before and during the war.

“It is quite regrettable to witness statues of comfort women being established or displayed in various parts of the world, which is incompatible with the position and measures taken by our government,” the Japanese official said.