Countries exempted from stiff U.S. tariffs on steel and aluminum will still face tough restrictions, like import quotas, White House trade adviser Peter Navarro reportedly said Tuesday.

"The guiding principle of this administration, from the president down to his team, is that any country or entity like the European Union, which is exempt from the tariffs, will have a quota and other restrictions," Navarro told steel executives at a meeting, according to Politico.

Such restrictions "are necessary to defend the aluminum and steel industries from imports in defense of our national security," Navarro said.

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Navarro's comments came shortly after President Trump moved to delay implementing steel and aluminum tariffs for several close U.S. trading partners.

Trump's plan calls for a 25 percent tariff on steel imports and a 10 percent tariff on aluminum.

The Trump administration pushed the decision to June 1 for the European Union (EU), Canada and Mexico, while reaching preliminary agreements with several other U.S. allies ahead of a looming midnight deadline.

Still, the restrictions outlined by Navarro are likely to upset officials in the EU, Canada and Mexico, who want full exemptions from trade barriers, because of their close relationships with the U.S.

The EU has already threatened $3.5 billion in retaliatory tariffs on U.S. consumer goods if Trump imposes duties on European goods.