(New throughout, adds background on trade case, China envoy, Mexico talks, updates share prices)
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.
A source close to the White House said recently Trump had expressed interest in imposing a tariff on steel imports of at least 24 percent.
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 Cold War-era national security law. The U.S. Commerce Department on Feb. 16 recommended stiff curbs on steel imports and offered several options ranging from global and country-specific tariffs to broad import quotas.
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.
These included the chief executives of Nucor Corp and ArcelorMittall, as well as the head of the United Steelworkers union, the people said.
Mike Bless, the CEO of Century Aluminum Co, is among those attending, a company official said.
One industry official said several chief executives who were invited would not be able to attend because they could not adjust their schedules on such short notice.
Automakers, which are major users of metals, and some administration officials were unclear about 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.
A Trump 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 his comment.
Top aides to Trump are due to meet Thursday with a senior envoy of Chinese President Xi Jinping at the White House.
Chinese economic adviser Liu He and his delegation will meet with U.S. Treasury Secretary Steven Mnuchin, U.S. Trade Representative Robert Lighthizer and White House economic adviser Gary Cohn.
Mexico's Economy Minister Ildefonso Guajardo met U.S. Commerce Secretary Wilbur Ross and Lighthizer during a hastily-arranged visit to Washington on Wednesday over concerns about possible U.S. tariffs on steel imports.
A blanket tariff would cover every steel and aluminum product entering the U.S. market from China.
U.S. steel stocks were broadly higher in early trading, with AK Steel up nearly 11 percent, U.S. Steel up more than 5 percent and Nucor Corp gaining 2.3 percent. Aluminum company Alcoa advanced about 1 percent in early activity.
The Commerce Department completed its so-called "Section 232" national security investigations on steel and aluminum in January and turned them over to the White House, without revealing its recommendations. (Reporting by Susan Heavey, David Shepardson and Eric Walsh Editing by Chizu Nomiyama and David Gregorio)