South Korea’s left-wing president Joon Mae-in has criticized a compensation deal between his country and Japan over the treatment of wartime sex slaves as “seriously flawed” and threatened to revoke it.

The agreement, which was signed in 2015 Japanese Prime Minister Shinzo Abe and Joon’s predecessor Park Geun-Hye, involved Japan apologizing to the victims of a sex slave operation known as the “comfort women,” and also providing a one billion yen ($8.8m) compensation fund. Both sides at the time agreed that the issue had been “irreversibly resolved.”

However, a state-appointed panel in South Korea determined that Japan had failed to properly communicate with the victims and that Japan had not publicised the fact they asked for omittance of the term “sexual slavery” and had demanded the removal of a number of statues honoring the women in Seoul, including one outside the Japanese embassy.

“This runs afoul of the established universal principle of the international community for settling history issues, and above all, it was a political agreement that excludes the victims themselves and citizens,” said President Moon in a statement.

“Along with citizens, I, as president, make it clear again that the comfort women issue can’t be settled through this deal,” he continued.

Japanese Prime Minister Abe was reported as saying the deal would “not be changed by even one millimeter,” while his foreign minister claimed voiding the deal would make relations between the two countries “unmanageable.”

“The Japan-South Korea agreement is between the two governments and one that has been highly appreciated by international society,” said Japan’s foreign minister Taro Kono. “If the South Korean government … tried to revise the agreement that is already being implemented, that would make Japan’s ties with South Korea unmanageable and it would be unacceptable.”

The issue is likely to push both countries towards a diplomatic impasse. Relations historically have been strained due to Japan’s 1910-1945 colonial rule over a formerly unified Korea, where an estimated 200,000 women were used as slaves in military brothels.

Yet relations have warmed in recent years amid a succession of like-minded governments, while both countries currently work together to counter the escalating security threat presented by North Korea.

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