Japan has conveyed its concern to the U.S. government that a statement made by Secretary of State Rex Tillerson could contradict an agreement with South Korea over the “comfort women,” sources close to the matter said.

Tokyo has told Washington that Tillerson’s words could give the impression that the U.S. government has expressed an understanding of South Korea’s stance. The U.S. side replied that it will take note of Japan’s position, the sources said Saturday.

“It’s one that only they can resolve,” Tillerson told reporters this month after a 20-nation ministerial meeting in Vancouver, British Columbia. “And we know that there’s more that needs to be done,” he said, referring to the issue of the Korean girls and women who were forced into Japan’s military brothels before and during the war.

The Korean Peninsula came under Japanese colonial rule in 1910.

The statement comes as South Korea has recently reignited tensions with Japan by requesting additional measures on the issue despite signing a 2015 bilateral agreement to “finally and irreversibly” resolve the protracted row. Tokyo has rejected the request.

The U.S. government sees the need for Japan and South Korea to bolster ties in the face of the rising threat posed by North Korea’s rapidly advancing nuclear and ballistic missile programs.

Tokyo has said it will work together with the United States and increase pressure on North Korea to the “maximum level.” But Tillerson’s statement raises the question of how close Tokyo and Washington are on the history-related issue, clouding trilateral relations as they deal with the North.

The Japanese government’s view is that the United States acknowledged the deal as the final solution to the comfort women issue after former Secretary of State John Kerry said in a statement after the signing of the agreement in 2015 that it clarified the two neighbors will finally and irreversibly resolve the matter.

Following Tillerson’s statement, Chief Cabinet Secretary Yoshihide Suga said at a news conference last week that the United States is “supporting the Japan-South Korean deal with consistency” and that Washington has not changed its view.