In March 2018, Trump authorized a 25% tariff on steel imports and a 10% duty on aluminum - import barriers that heralded the start of his administration's hawkish push on trade - after a government report found that foreign shipments of the metals imperil national-security interests.
The administration said the moves were necessary because United States trading partners were acting unfairly to disadvantage both the country's traditional economic pillars as well as its best hopes for future prosperity.
He added that Brazil and Argentina were "presiding over a massive devaluation of their currencies".
"Effective immediately, I will restore the Tariffs on all Steel & Aluminum that is shipped into the USA from those countries".
He then directed U.S. Trade Representative Robert Lighthizer to negotiate with countries seeking to turn their temporary tariff exemptions into permanent ones.
"I am nearly convinced that he will hear us", he said.
Brazil's Steel Institute said it was perplexed by Trump's decision.More news: Qualcomm unveils Snapdragon 865: Everything we know so far
"There is no initiative by the government to artificially devalue the Real, and the decision to tax Brazilian steel as a way to compensate American farmers is a retaliation against Brazil, which is inconsistent with the partnership relationship between the two countries", it said.
Across the country, farm bankruptcies have surged 24% since September 2018, a few months after U.S. trade disputes with China and other countries led to higher tariffs on key farm goods including soyabeans, cotton and dairy, according to analysis by the American Farm Bureau Federation.
The eighth largest economy in the world, Brazil, has survived a severe financial crisis from 2014 to 2016, but is now suffering from recession and stagflation.
President Donald Trump released a clutch of protectionist trade actions on Monday aimed at long-time USA allies, including some countries that have been close to the Trump administration.
Trump argued that move was negatively affecting American farmers and manufacturers by making it hard for them to fairly export their goods.
While speaking outside the White House before departing for London Monday, President Trump said steel companies and farmers will be "very happy" with the move.
Brazil has teetered on the brink of recession this year, and Argentina is again enmeshed in an economic crisis, which has led to the currencies of both countries weakening against the United States dollar.
The U.S. has imported about 3.8 million metric tons of steel from Brazil so far this year, most of which are slab, according to U.S. Census Bureau data.More news: Nunes Launches $435 Million Lawsuit Against CNN Over Ukraine Story
But as the global economy slows, the USA dollar tends to strengthen as it becomes a safe haven for nervous investors around the world.
Jose Urtubey, spokesman for Argentina's powerful UIA industrial lobby, said producers in the country will be harmed immediately by the tariffs.
Both South American nations were among a group of US allies that Trump had exempted from steel and aluminum tariffs in March 2018.
The head of Argentina's only aluminum producer, Javier Mandanes Quintanilla of the Aluar Group, viewed the tariff plan with trepidation. "We do not yet know the magnitude (of the measure)". The South American nation's steel accounts for about 3.5% of the 110 million tons consumed in the USA a year.
"Their economy is not comparable with ours; it's many times bigger".
Trump also urged the Federal Reserve to lower interest rates so countries "no longer take advantage of our strong dollar".
"Lower Rates & Loosen - Fed!" he admonished the independent agency.
But it has signaled it will stand back before deciding on any further moves.More news: TikTok is being sued for allegedly sending USA user data to China
Argentine Production Minister Dante Sica said Trump's announcement was "unexpected", and he was seeking talks with USA counterparts.