U.S. stocks dipped on Tuesday, a day after Alcoa posted in-line earnings but lower-than-expected revenues, as investors sought more evidence to justify the lofty levels on Wall Street.

Alcoa Inc , a Dow component, fell 2 percent to $14.29 after the aluminum producer reported that results benefited from higher prices but warned it could face strikes at its U.S. operations as earnings season began.

You get your feet wet, and Alcoa was weaker on revenue, said Ryan Detrick, senior technical strategist at Schaeffer's Investment Research in Cincinnati, Ohio.

(Overall corporate) earnings will probably beat, but the bottom line is the revenue numbers. That's what we want to see, some impressive growth, and that is what we are going to focus on without question as we continue to digest earnings the next couple of weeks.

The Dow Jones industrial average <.DJI> dropped 15.34 points, or 0.14 percent, to 10,990.47. The Standard & Poor's 500 Index <.SPX> shed 3.91 points, or 0.33 percent, to 1,192.57. The Nasdaq Composite Index <.IXIC> fell 4.15 points, or 0.17 percent, to 2,453.72.

The S&P 500 has risen nearly 40 percent over the past year and is up more than 76 percent from the 12-year lows reached in March.

After the markets close on Tuesday, Intel Corp is expected to report strong earnings, possibly offering new evidence that demand for electronics and personal computers is improving. Intel shares gained 0.6 percent to $22.67 ahead of the results.

CSX Corp , the No. 3 U.S. railroad, is due to report stronger earnings later Tuesday as the economy, helped by the auto and coal industries, gains momentum.

(Reporting by Chuck Mikolajczak; editing by Jeffrey Benkoe)

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