President Donald Trump cranked up the pressure on two of America’s key trading partners, Japan and South Korea, firing off letters Monday to the heads of their countries that informed them of their new tariff rate.

President Donald Trump said he’ll impose tariffs of 25% on goods from Japan and South Korea beginning Aug. 1, giving the two export powers a narrow three-week window to open their markets to American products and manufacture more in the US to avoid his unilateral levies.

The tariffs on two of America’s closest allies would go into effect Aug. 1, unless the countries reach some kind of trade agreement with the United States.