Chinese restrictions on the export of two key materials for the high-tech and defense industries could be a harbinger of problems for Western makers of electric vehicles. 

China’s Commerce Ministry on Monday said it would require permits for the export of gallium and germanium beginning next month. The timing of the announcement, just ahead of U.S. Treasury Secretary Janet Yellen’s arrival today in Beijing, underscored the chilly nature of U.S.-China relations and served as a warning of potential future actions. In an interview published Wednesday in the official China Daily, former Vice Commerce Minister Wei Jianguo, now vice chair of the China Center for International Economic Exchanges, a state-backed think tank, wrote, “This is just the beginning of China’s countermeasures, and China’s tool box has many more types of measures available.”