"The economy is worldwide now instead of just the U.S. China is going to attach something else if they bring in and it's just going to go round and round with them", Keith said.
For his part, Trump on Monday appeared to suggest that Canada and Mexico could win exemptions to his planned sweeping tariffs on steel and aluminum if the two countries sign a new NAFTA trade deal and take other steps.
On Thursday, Trump said the United States would apply duties of 25 percent on imported steel and 10 percent on aluminum to protect domestic producers.
Both Democrats and Republicans slammed him for the plan, which, critics say, could hike consumer prices and harm US manufacturers who use the imported materials.
Trudeau stressed in his comments he was prepared to "defend Canadian industry" and that the tariffs would also hurt USA consumers and businesses because prices could rise.
"We will not sit idly when European industry and jobs are threatened", Juncker said on the sidelines of a conference in Hamburg, Germany.
The United States imposes a 2.5-per cent tariff on cars assembled in Europe and a 25-percent tariff on European-built vans and pickup trucks.
Ross called the possible European levies a "pretty trivial amount of retaliatory tariffs, adding up to some $3 billion of goods".
China's Zhang Yesui, spokesperson for China's National People's Congress, said China would take "necessary measures" if its interests were hurt.
Joshua Bolten, who was chief of staff to President George W. Bush, said: "Nobody wins a trade war, especially in these globalilzed days".
On Friday, European Commission chief Jean-Claude Juncker said the EU was drawing up measures against leading U.S. brands such as Levi's and Harley-Davidson. "It's some Dollars 3 billion-odd of goods that the Europeans have threatened to put something on", Ross said.
European Union trade chiefs could apply 25% tariffs on around $3.5bn of imports from the USA - targeting iconic U.S. exports including Levi's jeans, Harley-Davidson motorbikes and Bourbon whiskey.
"We can't really have a country without a solid steel industry and without a solid aluminum industry, and right now, those industries are under siege", Navarro said in an interview on Breitbart News Saturday with Washington Political Editor Matthew Boyle on SiriusXM Patriot 125.