He justified the move saying those countries' weak currencies had made it harder for US food exports to compete.

"Brazil and Argentina have been presiding over a massive devaluation of their currencies, which is not good for our farmers," 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.

Key sector

Argentine production minister Dante Sica said he would also request a conversation with his US counterparts.

Despite Mr Trump's claims, both countries have taken steps over the past two years to try to bolster their currencies, which have dropped sharply amid domestic political turmoil and economic woes.

The plans for the tariffs, which Mr Trump announced on Twitter, were a surprise.

While Mr Trump last year imposed tariffs of 25% on steel and 10% on aluminium on national security grounds, he granted some exceptions amid outcry abroad and in the US, where many manufacturers rely on foreign metal imports.

Argentina and Brazil, which agreed to limits on their metals exports, were among the countries to receive waivers.

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.

The White House provided few details about the plans for the tariffs.

Analysts said such measures raise legal questions since Mr Trump is acting so long after the initial tariff decision.

However, for Mr Trump, farmers in rural states are an important block of voters, and they have had a difficult time in the past year.

Countries around the world retaliated with higher import duties on US farm exports in response to Mr Trump's metals tariffs.

China has also hit farm goods with tariffs as part of the US-China trade fight over policies related to technology and government subsidies. Meanwhile, Chinese buyers have looked to Brazil and Argentina as an alternative.

Across the country, farm bankruptcies surged 24% in the 12 months to October 2018, according to analysis by the American Farm Bureau Federation.

Analysis:

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.