Washington • President Donald Trump will sign a pair of proclamations Thursday that impose tariffs on imported steel and aluminum but offer relief to some U.S. allies.

The tariffs, which take effect in 15 days, initially will not apply to imports from Canada and Mexico so that U.S. officials can assess progress toward a new North American Free Trade deal, according to a senior administration official.

Other countries with a "security relationship" to the United States may seek exemptions by opening talks with the administration on "alternative ways" to address the threats the administration alleges their products pose to national security, the official said.

The plan differs significantly from Trump's initial suggestion he would apply the taxes to all imports, a change that follows a week of pleas from his fellow Republicans to narrow the tariffs or abandon them entirely.

The initial exemptions for Canada and Mexico are not time-limited but are designed to last only long enough for the administration to assess the outlook for the NAFTA renegotiations.

For countries that do not win exemptions, steel imports will be taxed at 25 percent and aluminum imports at 10 percent. If Mexico and Canada win permanent exemptions, tariffs on imports from other nations may have to be increased beyond 25 percent, the official said.

Trump plans to formally unveil the order in a White House event, and he previewed the remarks earlier Thursday.

"I'll have a right to go up or down depending on the country, and I'll have a right to drop out countries or add countries," Trump said in an appearance alongside his Cabinet. "I just want fairness because we have not been treated fairly by other countries."

The president championed the tariffs as necessary protections for two industries — steel and aluminum — that he called "the backbone of our nation."

"We're going to be very fair, we're going to be very flexible, but we're going to protect the American worker, as I said I would do in my campaign," Trump said.

Republicans have been pushing Trump to back off his planned tariffs since he shocked the party last week by announcing they were coming. A temporary exemption for NAFTA members would mark progress in that effort, but lawmakers are pressing the president to pull back further.

"If they were to exempt Mexico and Canada while other negotiations were ongoing, it would be a welcome gesture on behalf of the president and certainly would be received well on Capitol Hill," Rep. Mark Meadows, R-N.C., an influential House conservative, said before Trump's announcement.

Republican lawmakers criticized the rollout, with Sen. John Thune, R-S.D., calling the process "chaotic and confusing."

Hours before Trump's promised announcement, foreign diplomats, industry leaders and Trump allies were still in the dark. Their outreach to senior administration officials seeking definitive answers was returned with shrugs or unanswered calls.

"The people in the building have no idea what's about to happen," said one Trump donor opposed to the tariffs and spoke on the condition of anonymity to be candid.

As can be Trump's wont, the president left Washington in suspense via an early morning post on Twitter. At 7:38 a.m. Thursday, he wrote: "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."

The meeting was not on the president's daily schedule, released to the public Wednesday night. Several hours after Trump's tweet, senior White House aides did not know whether the announcement would happen Thursday. Trump dispelled those doubts later that morning during the Cabinet meeting, when he again touted the tariffs and previewed his afternoon announcement.

The White House was also being lobbied heavily against the tariffs by outside conservative groups that have worked with the administration on other issues, including passing the new tax cut law.

Tim Phillips, president of Americans for Prosperity, an advocacy group backed by the billionaire GOP donor Koch brothers, warned in an interview that the tariffs could harm Republican prospects in the November midterms when Democrats hope to retake control of Congress.

"If American consumers are paying higher prices on a wide range of products just as the economy is starting to take off, it can slow the economic recovery," Phillips said. "It could definitely impact the midterms."

Phillips said that merely narrowing the tariffs to exclude Mexico, Canada or other allies would not be enough to alleviate his group's concerns.

"The administration can still not do this, and we're urging them not to do it, and it's an ongoing conversation," he said.

The Washington Post's Josh Dawsey contributed to this report.