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Trump said on Thursday the duties, 25 percent on steel imports and 10 percent on aluminium, would be formally announced next week, although White House officials later said some details still needed to be ironed out.
"We are discussing different measures. Everything from bringing the case to the WTO (World Trade Organisation), alone or with partners who are also affected, and also safeguard measures and possible retaliation," Malmstrom said.
"These are things we are discussing internally in the (EU)Commission and with our member states. But obviously nothing will be announced until we know the full extent of these measures."
Earlier on Friday, Australia's trade minister also said the planned tariffs risk retaliation from other economies and could cost jobs, while China predicted harm to trade if other countries follow the example of the U.S.
Fears of an escalating trade war roiled Asian markets, hitting the share prices of steelmakers and manufacturers supplying U.S. markets particularly hard.