President Trump on Monday said the U.S will impose 25% tariffs on goods from South Korea and Japan by Aug. 1, while later threatening to hit more than a dozen other nations with steep import duties by next month.

Trump uses tariffs as a key tool to force changes to U.S. trade relationships, but his on-again-off-again approach has roiled markets and stoked uncertainty.

President Trump has told 14 countries that they will face tariffs of at least 25 percent on Aug. 1 if they don’t reach agreements by then.