The trip came after tensions between Seoul and Tokyo spiked as Japan toughened restrictions on exports to South Korea of key industrial materials used in semiconductors and displays in an apparent retaliatory measure in a separate row over Japan's wartime forced labor.

"I met with people in the US administration, Congress and think tanks, and explained the unfairness of Japan's unilateral measure and the effects of this unfair measure have on the security situation among the South, the US and Japan," Kim told reporters at Washington's Dulles International Airport.

"All the people I've met shared the understanding about the concerns over tensions between the South and Japan caused by this unilateral measure," Kim said. "That's why the State Department said it's going to do everything it can to maintain cooperation among the South, the US and Japan."

Kim said, however, that US officials have not talked explicitly about "mediation."

"I believe that the US is going to do what it has to do if it considers cooperation among the South, the US and Japan important," he said.

Japan imposed the export curbs on July 4, in an apparent retaliatory step for last year's Supreme Court rulings here that ordered Japanese firms to compensate victims of wartime forced labor.

Japan has protested the rulings, arguing that all reparation issues stemming from its 1910-45 colonization of the peninsula were settled under a 1965 government-to-government accord aimed at normalizing bilateral ties.

South Korea has refused to intercede in the civil litigation, saying it honors the democratic constitutional principle that guarantees the separation of the executive, legislative and judicial powers.