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.

Major indexes deepened losses in afternoon trading on Monday, with the Dow falling over 500 points as investors digested the latest trade updates.

Financial markets are on edge about higher import taxes, but White House officials say they anticipate “a lot of deals very quickly.”