U.S. President Donald Trump substantially raised tariffs on steel and aluminum imports on Monday to a flat 25% "without exceptions or exemptions" in a move to aid the struggling industries but which increases the risk of a multifront trade war.

Trump signed proclamations raising the U.S. tariff rate on aluminum to 25% from his previous 10% rate and eliminating country exceptions and quota deals as well as hundreds of thousands of product-specific tariff exclusions for both metals. A White House official confirmed that the measures would take effect on March 4.

The tariff rate will rise back to 25% on millions of tons of steel and aluminum imports from Canada, Brazil, Mexico, South Korea and other countries that had been entering the U.S. duty free under the carve-outs.