(Adds AP report)
* Trump says he will stick to planned tariff levels
* AP says Canada, Mexico exempted indefinitely
* Europe has "whole arsenal" for any trade war - official
* EU could target goods produced in Republican-ruled states
* Beijing calls trade war a "mistaken prescription"
WASHINGTON/SANTIAGO, March 8 (Reuters) - President Donald Trump pressed ahead with the imposition of 25 percent tariffs on steel imports and 10 percent on aluminum on Thursday but exempted Canada and Mexico, backtracking from earlier pledges of tariffs on all countries.
Details of the plan came from a briefing by administration officials ahead of Trump's speech, which had been due to start at 3:30 p.m. (2030 GMT). Trump will say that other countries can apply for exemptions, according to the administration, although details of when they would be granted were thin.
Trump has offered relief from steel and aluminum tariffs to countries that "treat us fairly on trade," a gesture aimed at putting pressure on Canada and Mexico to give ground in separate talks on the North American Free Trade Agreement (NAFTA), which appear to be stalled.
Trump has also demanded concession from the European Union, complaining that it treated American cars unfairly and has threatened to hike tariffs on auto imports from Europe.
Stock markets in Canada and Mexico rallied on the news, as did the Canadian dollar and the Mexican peso.
There was no mention of Mexico and Canada giving ground on NAFTA in the proposals.
Trump's tariffs have triggered the threat of countermeasures from the European Union and now China. The levies aim to hit Beijing, although China exports very little of either metal to the United States.
(Additional reporting by Michael Martina, Elias Glenn, Kim Coghill, Brian Love, Nichola Saminather, Doina Chiacu and Andrea Hopkins; writing by David Stamp and David Chance; editing by Jonathan Oatis and Frances Kerry)