Japan decided early this month to remove South Korea from its "whitelist" of trusted trading partners, and the decision took effect Wednesday (Seoul time), adding to the already high tensions in their tit-for-tat row stemming from differences over their shared history.

"The ROK and Japan must resolve these sensitive matters," a department spokesperson told Yonhap News Agency. "The United States, as a close friend and an ally to both, will do what it can to support their efforts to resolve this."

The Seoul-Tokyo row has increasingly posed a challenge to Washington, which sees trilateral security cooperation as critical to efforts to confront North Korea's nuclear threats and China's growing military assertiveness.

"As an ally and friend to both Japan and the Republic of Korea, the United States believes it is critical to ensure strong and close relationships between and among our three countries in the face of shared regional challenges, including those posed by the DPRK, as well as our other priorities in the Indo-Pacific and around the world," the spokesperson said, using the initialism of North Korea's official name, the Democratic People's Republic of Korea.

"The United States will continue to encourage our two close allies to resolve this issue through sincere discussions," the spokesperson added. (Yonhap)