"We will now trigger a dispute settlement case at the WTO since the USA measures on steel and aluminum clearly go against agreed worldwide rules", she said.
"We look forward to continued negotiations, both with Canada and Mexico on the one hand, and with the European Commission on the other hand, because there are other issues that we also need to get resolved", Ross told the reporters in a telephone briefing on Thursday.
He told Radio 4's Today programme: "The president has said that he believes he could win a trade war, we think that a trade war is in nobody's interests".
Cecilia Malmstrom, the European Union trade commissioner, announced the European Union would be seeking to implement a number of retaliatory tariffs on American products by June 20.
Brussels is also preparing to slap tariffs on USA products including bourbon, motorcycles and blue jeans worth up to €2.8 billion (S$4.4 billion).
Trump had originally imposed the tariffs in March, saying a reliance on imported metals threatened national security.
European Union trade commissioner Cecilia Malmstrom announced the bloc would take the USA to the World Trade Organisation to challenge the legality of the new tariffs, saying negotiations with Washington were not now possible.
Previously Brussels pledged to retaliate the U.S. protectionist steps with its own 25 percent tariffs on USA products, including motorcycles, jeans cigarettes, cranberry juice and peanut butter.
Germany's carmakers are especially braced for the latest threat from Trump, who earlier this month launched proceedings that could eventually slap 25 per cent tariffs on auto imports.
He continued: "The reason I don't know is because he's talked about the fact of this, he's anxious about trade surpluses, trade deficits around the world. So, we will immediately introduce a settlement dispute with the WTO and will announce counter-balancing measures in the coming hours", Juncker said in a speech in Brussels. "From the beaches of Normandy to the mountains of Afghanistan, we have fought and died together", Trudeau said in a Thursday speech.
He was speaking a day after International Trade Secretary Liam Fox refused to rule out a trade war after the USA imposed tariffs of 25 percent on steel and 10 percent on aluminum imported from the EU, Canada and Mexico.
USA companies, too, have warned about the negative impact of the tariffs which will raise their costs.
The United States was singled out by some of its closest allies Saturday over the imposition of tariffs that they warn will undermine open trade and weaken confidence in the global economy.
She said the measures "will cause a lot of damage to our steel and aluminum industry" and risk hurting global economic growth.
She said: "This is further weakening the transatlantic relations and it also increases the risk of severe turbulences in the markets globally". "It's Donald Trump's failure to understand the implications for the United States and world economy of his isolationist policies".
The EU earlier threatened to counterpunch by targeting US products, including Kentucky bourbon, blue jeans and motorcycles.