Mr Trump opened up a new front in his trade war by tweeting that he would reinstate tariffs, effective immediately, on all steel and aluminium shipped into the United States by the two South American nations.
In a tweet posted just after 6 a.m., Trump said the countries have been "presiding over a massive devaluation of their currencies" which he says is bad for American farmers.
At its most recent meeting, in October, the Federal Reserve cut short-term interest rates for a third time this year to try to support the USA economy.
Both South American countries have benefited from the US trade war with China.
Both Brazil and Argentina were exempted from 25% steel and aluminum tariffs previous year when Trump was attempting to avoid a trade war with those countries.
Latin America's third-largest economy, Argentina, is now facing a deepening economic crisis which has forced the government to impose currency controls in a bid to stabilize financial markets. "This makes it very hard for our manufactures & farmers to fairly export their goods. Lower Rates & Loosen - Fed!" Macri lost his re-election bid in October and will leave office on December 10.
The Reserve Bank board will meet today but most economists are not expecting an interest rate cut at 2:30pm (AEDT).
In another missive Monday morning, Trump said: "U.S. Markets are up as much as 21% since the announcement of Tariffs on 3/1/2018 - and the U.S.is taking in massive amounts of money (and giving some to our farmers, who have been targeted by China)!"
After a brief spike in steel prices, US steel producers have struggled since the Trump administration put tariffs into place previous year.
U.S. Steel Corp. was up 1.8 percent, and Steel Dynamics Inc. gained 1 percent. Shares of AK Steel are down 10%, even as the S&P 500 sets record highs repeatedly.