WASHINGTON, March 8 (Xinhua) -- U.S. President Donald Trump on Thursday formally signed proclamations to impose steep tariffs on imported steel and aluminum amid mounting dissent from business groups and trading partners around the world.

The United States will impose a 25-percent tariff on imported steel and 10-percent on aluminum, Trump said at an event at the White House, noting a strong steel and aluminum industry is "vital to our national security."

The tariffs will take effect in 15 days with initial exemptions for Canada and Mexico pending the renegotiation of the North American Free Trade Agreement (NAFTA).

"We're going to hold off the tariff on those two countries, to see whether or not we're able to make the deal on NAFTA," Trump said.

Trump signaled that all other countries also have opportunities to be exempt from the tariffs by negotiations with the United States.

"If the same goals can be accomplished by other means, America will remain open to modifying or removing the tariffs for individual nations, as long as we can agree on a way to ensure that their products no longer threaten our security," he said.