Jean-Claude Juncker, the president of the European Commission, said the EU would take retaliatory action if Trump followed through and Canada said that any trade impediments would be "absolutely unacceptable".
Minister Hiroshige Seko told a news conference his US counterpart Robert Lighthizer had only explained the schedule and procedure of the USA actions in talks in Brussels.
Any trade war with the United States will only bring disaster to the world economy, Chinese Commerce Minister Zhong Shan said on Sunday, as Beijing stepped up its criticism on proposed metals tariffs by Washington amid fears it could shatter global growth.
"We are an ally, not a threat", he said, rejecting any hint that the bloc's exports threatened U.S. national security - Trump's justification for imposing the tariffs.
Brussels is also looking to "safeguard" measures to protect its industry - restricting the bloc's imports of steel and aluminium to stop foreign supplies flooding the European market, which is allowed under World Trade Organisation rules.
"Europe is certainly not a threat to American internal security so we expect to be excluded", European trade commissioner Cecilia Malmstrom said in Brussels.
Trump signed the blanket tariffs - 25% on steel imports, 10% on aluminium imports - following a meeting with United Steelworkers union representatives at the White House on Thursday.
"The European Union, wonderful countries who treat the US very badly on trade, are complaining about the tariffs on Steel & Aluminum", Trump tweeted.
"Expectations from the steel and aluminum industries were high headed into yesterday's announcement, and Trump's late-stage decision to offer specific exemptions to both Canada and Mexico and possible exemptions to other allies meant the final tariffs failed to meet them", Allen said.
As talks to renegotiate the trade deal enter an expected eighth round in the coming weeks, the think tank says in a new report that would be the best-case scenario in a post-NAFTA world.
The EU exported about 5.5 million tons of steel to the US past year.
He says they're working "very quickly" on an agreement that would exempt Australia from tariffs on steel and aluminum imports Trump ordered this week.
The spokesperson also said the WTO has fielded some informal concerns from countries about Trump'stariffs, but no formal complaint.
Other officials at the EU, by far the biggest trading partner of the United States by value, have warned it could take counter-measures including European tariffs on U.S. oranges, tobacco and bourbon.
Major producer Brazil immediately vowed to take "all necessary steps" in order to "protect its rights and interests" in response to the USA move.
"We are prepared and will be prepared if need be to use rebalancing measures", he said.
And while the full economic impact remains unknown, the political fallout was swift with the top Republican in Congress Paul Ryan publicly denouncing Trump's move, and vowing to push him to narrow its focus to "countries and practices that violate trade law".
A Wrinkle in Time
Even when working with a major studio and a major property, she has made a movie that's completely her own, flaws and all. With that film, DuVernay became the first African-American female director ever to be nominated for a Golden Globe .
Ahead of Macron-Modi's UP visit, CM checks preparations
French President Emmanuel Macron on Saturday said that France should be the best partner for India and the entry point to Europe . During the visit, the two countries would look to boost ties, especially in the areas of maritime security and counter-terrorism.