South Korean President Moon Jae-in urged Tokyo on Thursday to act on the basis of remorse and reconciliation when managing bilateral ties and acknowledge the historical truth about women forced to provide sex for Japanese troops before and during World War II.

Amid tension between Seoul and Tokyo over the future of a 2015 bilateral agreement on the “comfort women” issue, Moon described the women’s treatment as an inhumane crime and said that Japan, “the perpetrator, must not declare (that the issue) is over.” Moon made the remarks in a speech at an annual ceremony commemorating a movement for Korean independence during Japanese colonial rule.

“I do not seek special treatment from Japan,” Moon said. “I ask only that (Japan) walk alongside us into the future on the basis of heartfelt remorse and reconciliation, befitting our closest neighbor.”

Moon’s administration has taken issue with the way in which the comfort women agreement was negotiated under his impeached predecessor Park Geun-hye, and has requested that the Japanese government take further action. Japan maintains the existing agreement is valid, noting that the issue being considered “finally and irreversibly” resolved was a condition of the pact.

In Tokyo, the government’s top spokesman said Japan had lodged a diplomatic protest over Moon’s remarks. Chief Cabinet Secretary Yoshihide Suga described Moon’s comments as “extremely regrettable.” Suga, speaking at a regular briefing, also urged cooperation between South Korea and Japan to tackle North Korea.

Under the 2015 deal, Japan paid ¥1 billion ($9.4 million) to a South Korean foundation set up to support former comfort women while South Korea said it would “strive to solve” the issue of a statue commemorating comfort women in front of the Japanese Embassy in Seoul that Japan wants relocated.

Speaking Thursday, Moon also reiterated South Korea’s sovereignty over a group of islets it administers in the Sea of Japan.

He said Japan’s continued claim over the islets amounts to a denial of its history. The rocky islets are called Dokdo in Korean and Takeshima in Japanese.

The government-organized ceremony was held in a former prison in Seoul to commemorate the nationwide independence movement that began on March 1, 1919. Japan colonized the Korean Peninsula from 1910 to 1945.

Yuki Asaba, professor and Korea expert at the University of Niigata Prefecture, said it was fair for Japan to strongly protest a speech that further discussed an issue previously said to have been “finally and irreversibly” resolved. But he added that South Korea’s intention may not necessarily have been to seek renegotiation or reversal of the 2015 agreement.

Asaba noted that Moon’s remarks are the “toughest” he has used toward Japan since Seoul announced in January a new policy on the 2015 agreement, which asked for a voluntary apology from Japan, and could be related to a historical view that South Korea’s roots go back to 1919 when a provisional government was launched there. Conservative lawmakers in South Korea claim that the country was founded in 1948, after World War II.

“If (Moon’s view) is that the country was founded in 1919,” that would mean he sees Japan’s occupation of Korea “as a forcible occupation,” Asaba said.

On Tuesday, the city of Seoul claimed that a team of South Korean scholars and researchers had discovered videos and photos of the scene of a 1944 massacre by Japanese troops of South Korean comfort women during World War II.

South Korean media said the video, which had been kept in the U.S. National Archives and Records Administration, was the first to be found that captured the massacre of comfort women.

The video and pictures appear to show corpses of women in a hole in the ground.

Seoul claimed that the videographer had been identified, but did not describe how it was determined that the victims were comfort women.