Bad day for world tade
European Commission President Jean Claude-Juncker responded to the announcement, saying the EU will impose countermeasures on the US.
"This is a bad day for world trade," Juncker said in a speech in Brussels.
"So, we will immediately introduce a settlement dispute with the WTO [World Trade Organisation] and will announce counterbalancing measures in the coming hours," he added.
"It is totally unacceptable that a country is imposing unilateral measures when it comes to world trade."
Al Jazeera's Peter Sharp, reporting from London, said "it's going to be a difficult time," adding that "over the next 24 hours, we will see ... if this is indeed going to be a trade war."
The newly announced tariffs could also hinder the negotiations between the US, Canada and Mexico on NAFTA.
Responding to the US tariffs, Mexico announced it will impose tariffs on US imports, including pork bellies, apples, grapes, cheeses and flat steel, among other products.
Fears of a global trade war are already weighing on investor confidence and could hinder the global economic upturn. European officials argue that tit-for-tat tariffs will hurt growth on both sides of the Atlantic.
Brazil, Argentina and Australia have agreed to limit steel shipments to the US in exchange for being spared the tariffs, the US Department of Commerce said. Tariffs will remain on imports from Japan.