NEW YORK, March 8 (Reuters) - U.S. President Donald Trump's import tariffs on steel and aluminum will start in 15 days with exemptions for Canada and Mexico for an undetermined period, and the possibility of alternatives for other nations, a senior administration official said on Thursday. Story: COMMENTS: BORIS SCHLOSSBERG, MANAGING DIRECTOR OF FX STRATEGY, BK ASSET MANAGEMENT, NEW YORK “This is not really tariff. It’s more theater. This is much tamer than what we had expected initially. The interesting open question is whether there is more to come. Will it become contentious with China and intellectual properties? That could be a more of a dangerous game to play. You would have the No.1 and No. 2 economies in the world going at it and become contentious? Will Trump go toe to toe with the Chinese?" MARK GRANT, CHIEF GLOBAL STRATEGIST AT B. RILEY FBR INC, FORT LAUDERDALE, FLORIDA: “I think it is a positive for two of our most important trading partners. But the news is problematic for the EU (European Union) so let’s see how they respond. Do I think this will turn into a trade war? No. Do I think there will be some retaliation? Yes. I do expect other tariffs over the next few months. I think both bond and stock markets were fearful initially that this would lead to a trade war and the end result is that things have certainly calm down. I honestly think President Trump is trying to strengthen America’s hand and will probably be doing more of that.” KIM FORREST, SENIOR PORTFOLIO MANAGER AT FORT PITT CAPITAL GROUP IN PITTSBURGH "My dad had a great saying – don’t cry until you are hurt. Apparently there are no real tariffs being put in place. It’s more of a bid to bring trading partners to the table." MARKET REACTION: STOCKS: U.S. stocks extended gains ahead of the announcement, as the AP reported key details. The S&P 500 was last up 0.2 percent. BONDS: U.S. treasury bond yields rose slightly, then dipped. FOREX: The Canadian dollar and Mexican peso gained against the U.S. dollar. The U.S. dollar index gained slightly then dipped.