WASHINGTON (AP) —  Sources outside the White House say President Donald Trump's tariffs on steel and aluminum imports will take effect in 15 days, with Canada and Mexico indefinitely excluded from the duties.

Trump is ordering import tariffs of 25 percent on steel and 10 percent on aluminum later Thursday. He says American manufacturers need to be protected on national security grounds.

The people briefed on the plans say all countries affected by the tariffs are being invited to negotiate for exemptions, if they can address the threat their exports pose to the U.S.

Those briefed on the plans say the exclusion for Canada and Mexico could be ended, if talks stall to renegotiate the North American Free Trade Agreement.

The people were not authorized to discuss details in advance and spoke on condition of anonymity.