(New throughout, adds details and background)
WASHINGTON, March 1 (Reuters) - U.S. President Donald Trump tweeted on Thursday that the U.S. steel and aluminum industries need "free, fair and smart trade" as a U.S. cable channel reported that he will make an announcement on tariffs at 11 a.m. (1600 GMT)
CNBC said an unidentified White House official informed it of the planned announcement, but provided no details on the tariffs.
Trump has vowed to take steps to crack down on imports of steel and aluminum and has been considering imposing hefty tariffs or quotas on imports of the metals from China and other countries under a national security law.
Trump has until April 11 to announce his decision on steel import curbs and until April 20 to decide on aluminum restrictions.
Several top U.S. steel and aluminum executives have been invited to the White House on Thursday ahead of what could be a major trade announcement, according to two people familiar with the meeting.
Automakers, which are major users of metals, and some administration officials were unclear what Trump planned to announce on tariffs.
Administration officials noted that Trump at times has announced plans for immediate economic actions only to have them delayed by other White House officials.
Most recently, a comment about a "reciprocal tax" last month set off sharp protests from foreign governments. A day later, the White House said no immediate policy change was coming despite Trump’s comments.
U.S. steel stocks were broadly higher in premarket trading, with AK Steel up 7 percent, U.S. Steel up more than 4 percent and Nucor gaining 3.5 percent. Aluminum company Alcoa advanced 1.8 percent before the opening bell. (Reporting by Susan Heavey, David Shepardson and Eric Walsh Editing by Chizu Nomiyama and David Gregorio)