This week Japan announced toughened controls on high-tech exports to South Korea amid an ongoing dispute over Korean court rulings against Japanese firms.

The courts ordered Japanese companies to compensate South Korean workers for their forced labor during Tokyo's 1910-45 colonial rule of the Korean Peninsula.

"As an ally and friend to both Japan and the Republic of Korea, 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," a State Department spokesperson said in response to a Yonhap query, referring to North Korea by its official name, the Democratic People's Republic of Korea.

"The United States is committed to strengthening further our trilateral cooperation with Japan and the Republic of Korea. We remain unified in pressuring North Korea to denuclearize."

Growing tensions between Seoul and Tokyo have been an ongoing source of concern for Washington as it tries to boost coordination against Pyongyang's nuclear threat.

"The United States always pursues ways to strengthen relations between and among our three countries, both publicly and behind the scenes," the spokesperson added.

US President Donald Trump traveled to both countries last week to attend a Group of 20 summit in Osaka, Japan, and then to meet with South Korean President Moon Jae-in in Seoul.

Trump also traveled to the inter-Korean border and held a surprise meeting with North Korean leader Kim Jong-un. (Yonhap)