Japan’s foreign minister pledged “necessary measures” against South Korea in a dispute over compensation for colonial-era forced labor that has raised tensions between the neighbors and spilled into trade relations.

Taro Kono, in a statement released by the foreign ministry, didn’t elaborate on what the actions would be. His comments came the day after a deadline set by Tokyo for Seoul to name an arbitrator for the dispute. Japan earlier this month placed export restrictions on specialized materials vital for South Korea’s tech industry.

Kono said recent South Korean court rulings holding Japanese companies liable to compensate Koreans who worked at colonial-era mines and factories violated a 1965 treaty forming the basis for their relations and spells out an arbitration process for disputes that aren’t settled diplomatically.

“Above all, the decisions completely overthrow the legal foundation of the friendly and cooperative relationship that Japan and the Republic of Korea have developed since the normalization of diplomatic relations,” he said.

Kono also urged South Korea to remedy the situation immediately, and summoned the country’s Tokyo-based ambassador in order to make a formal protest.

Long History

The two U.S. allies have increasingly been at loggerheads as flaring tensions over Japan’s 1910-1945 colonization of the Korean Peninsula threaten to damage economic and defense ties that had previously seemed immune.

South Korea has proposed setting up a joint fund for compensation, but Japan said that violates the treaty terms and international law. The proposal wasn’t received well in Tokyo, after President Moon Jae-in’s government shut down a separate fund meant for women trafficked in Japanese imperial army brothels.South Korean courts have said that the individuals involved in the forced labor cases have not been compensated for their pain and suffering.

Japan’s tighter checks on exports—set to be expanded as soon as next month—may hurt the economy, and Moon is seeking an extra budget to offset any effects. But there is little incentive for either government to reduce tensions at present.

Support for Moon rose to 48% this week from 45% a week ago, with respondents to a Gallup poll citing approval for his diplomacy as the reason. Japanese media surveys have also shown widespread approval for Prime Minister Shinzo Abe’s handling of disputes with South Korea, including the export controls.