Japan's decision to curb high-tech material exports to South Korea, announced this week, had been decided upon as early as May, according to a Japanese media report.

According to the Yomiuri Shimbun on Tuesday, Tokyo decided to restrict South Korea-bound exports in May and a very small number of government officials chose the items to be restricted.

The report says the restrictions were considered along with a range of other retaliatory measures in response to South Korean Supreme Court rulings last year ordering Japanese companies to compensate Korean victims for wartime forced labor during Japan's colonial rule. 

On Monday, Japan announced it would strengthen regulations on exports of fluorinated polyimides, photoresists, and etching gas to South Korea starting Thursday. 

The highly-specialized products are used to produce semiconductors and displays. Japan controls 70 to 90 percent of global production of these inputs, and the restrictions represent a significant obstacle for Korea’s tech industry.

According to Yomiuri, officials from Japanese Prime Minister Shinzo Abe’s office supported export controls despite concerns that they may harm Japanese exporters. Japan also reportedly considered tightening issuance of visas for South Koreans. 

Another Japanese daily, the Mainichi Shimbun, reported the export curbs are a warning to South Korea that diplomatic or legal disagreements may be met by other retaliatory measures in the future, such as restriction on remittances and tighter visa issuance.