(CNN) Confusion abounded late Wednesday in the West Wing over President Donald Trump's impending trade announcement rolling out tariffs on steel and aluminum imports.

White House aides had been told to prepare for a signing ceremony Thursday at 3:30 p.m. ET in the Roosevelt Room, where steel and aluminum workers also are set to attend. But one White House aide said Wednesday night that the event had been canceled -- because the policy was not yet finalized.

As of late Wednesday evening, the tariff announcement was not listed on the White House schedule.

White House trade adviser Peter Navarro said in a TV interview Wednesday night that Mexico and Canada would be exempt from the tariffs, reinforcing the possible exemption that press secretary Sarah Sanders had mentioned at the White House briefing in the afternoon.

"There are potential carve-outs for Canada and Mexico based on national security, and possibly other countries as well," Sanders had said. She said the national security exemption would be on a "case-by-case" and "country-by-country" basis.

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