US President Donald Trump has said he will restore tariffs on steel and aluminium imports from Brazil and Argentina.
He suggested the countries’ weak currencies and cheap exports were harming US farmers.
“Brazil and Argentina have been presiding over a massive devaluation of their currencies,” Mr Trump said.
Brazil’s President Jair Bolsonaro said he would seek talks with Mr Trump.
“Their economy is not comparable with ours, it’s many times bigger. I don’t see this as retaliation,” Mr Bolsonaro said in a radio interview with Brazil’s Radio Itatiaia.
“I’m going to call him so that he doesn’t penalise us. Our economy basically comes from commodities, it’s what we’ve got,” he said.
Argentine production minister Dante Sica said he would also request a conversation with his US counterparts.
Mr Trump originally announced the 25% tariffs last year, citing national security grounds which have been contested in the US and abroad.
He then relented to lobbying from free trade advocates for some US allies, including Argentina and Brazil.
The US is Brazil’s biggest customer for steel and Brazil is the world’s 10th biggest exporter of steel, according to the US Department of Commerce.
Steel accounts for 3.7% of the total goods Brazil exported in 2018.
For Mr Trump, farmers in rural states are an important block of voters, and they have had a difficult time in the past year.
Across the country, farm bankruptcies have surged 24% since September 2018, a few months after US 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.
By Daniel Gallas, BBC South America business correspondent
Mr Trump accused Argentina and Brazil of “presiding over a massive devaluation” – with exports of both countries being now cheaper and more competitive due to the weak currency.
But Argentina and Brazil are not happy with the devaluation and both central banks have been intervening to prop up the sliding currencies.
Brazil’s currency, the real, has reached an all-time low, as investors feel uncertain about sluggish economic recovery and the future reforms promised by the government.
In Argentina, investors are on the sidelines waiting for a new direction to the economy, which will be set by president-elect Alberto Fernandez when he takes office next week.
This crisis will be a test for Brazil’s president Jair Bolsonaro, often called “Trump of the Tropics”, who boasts a close relationship with his US counterpart.