President Donald Trump said he will make his announcement on steel and aluminum tariffs at a 3:30 p.m. ET meeting Thursday.

The announcement comes a week after Trump said he would institute a 25 percent tariff on steel imports and a 10 percent tariff on aluminum. The move rattled markets as it indicated the president was moving toward more protectionist trade policies.

A White House official, who declined to be named, told CNBC the president on Thursday will sign a "legal paper enacting the president's stated objectives with dates of enactment and implementation included." Earlier Thursday, NBC News reported that if Trump were to sign a proclamation Thursday it would be largely "symbolic."

Markets are watching for more concrete signs that Mexico and Canada could be exempted from the tariffs. The White House has said the president was considering such exemptions.

Two sources familiar with the planning told CNBC that Trump is considering a 30-day exemption for Canada and Mexico, which would be tied to negotiations on the North American Free Trade Agreement. The president has repeatedly threatened to pull out of the 1994 pact if negotiators do not figure out a way to overhaul it.

Thursday morning, Trump tweeted: "Looking forward to 3:30 P.M. meeting today at the White House. We have to protect & build our Steel and Aluminum Industries while at the same time showing great flexibility and cooperation toward those that are real friends and treat us fairly on both trade and the military."

TRUMP TWEET

— CNBC's Eamon Javers and Kayla Tausche contributed to this report.