US President Donald Trump on Monday announced plans to reimpose tariffs on steel and aluminum from Brazil and Argentina, hitting back at what he called their "unfair" policies.

The move appeared to surprise his Brazilian counterpart, Jair Bolsonaro, who considers himself an ideological ally of the Republican leader. Industry leaders in both countries cried foul.

"Brazil and Argentina have been presiding over a massive devaluation of their currencies," which is hurting American farmers, Trump said on Twitter.

"Effective immediately, I will restore the Tariffs on all Steel & Aluminum that is shipped into the US from those countries."

Trump last year announced global tariffs of 25 per cent on steel and 10 per cent on aluminum but later approved exemptions for some countries, including Argentina and Brazil -- after they agreed to quotas.

Bolsonaro sought to play down the issue, saying he would appeal to Trump for more understanding of Brazil's position and boasting he has an "open channel" with the US leader if needed.