** Shares of steel and aluminum manufacturers in the United States rise premarket after President Donald Trump raises tariffs on steel and aluminum imports to a flat 25% "without exceptions or exemptions"

** United States Steel X.N up 1%, Nucor NUE.N and Steel Dynamics STLD.O rise more than 2% in premarket trade

** Roughly a quarter of all steel used in the U.S. is imported, mostly from neighbors Mexico and Canada or close allies in Asia and Europe like Japan, South Korea and Germany

** Aluminum companies Century Aluminum CENX.O and Alcoa AA.N up more than 1% premarket

** Roughly half all aluminum used in the U.S. is imported, with the vast majority coming from Canada

(Reporting by Anshuman Tripathy)

((Anshuman.Tripathy@thomsonreuters.com))