South Korean President Moon Jae-in has said that Japan’s reported accusations that Seoul has violated international sanctions by exporting banned goods to North Korea posed a “grave challenge” to his country.

Speaking at a meeting with senior aides on Monday, Moon warned that Japan’s recent imposition of curbs on exports of high-tech materials to South Korea would inflict greater damage on the Japanese economy. The president also urged Tokyo to return to talks.

Japan has tightened restrictions on the export of three materials used in high-tech equipment, citing “inadequate management” of sensitive items exported to South Korea.

The curbs were seen as a response to South Korean court rulings ordering Japanese companies to compensate wartime forced laborers, Reuters said.