U.S. stocks opened mostly flat on Wednesday, as investors awaited a key speech from U.S. Federal Reserve's Chair Janet Yellen for more clues on the timing of the next interest rate hike.

Traders kept a close eye on Yellen, who is scheduled to deliver a speech Friday on the U.S. economy and monetary policy at the Economic Policy Symposium in Jackson Hole, Wyoming.

"Will the Fed raise rates on September 21? Market probability is now 28 percent, up from near zero in June, but not up much," said Chris Low, chief economist at FTN Financial, on Wednesday.

However, some hawkish comments by Fed officials have strengthened market expectations that the central bank might raise interest rates later this year.

Trading volumes have been below average recently as the earnings season winds down, according to Reuters.

"Markets are quiet in part because it' s August. Stuff still happens in the summer, of course, but members of the press are on collective vacation," said Low.

Shortly after the Friday' s opening bell, the Dow Jones Industrial Average shed 12.26 points, or 0.07 percent, to 18,535.04. The S&P 500 ticked down 0.70 points, or 0.03 percent, to 2,186.20. The Nasdaq Composite Index edged up 2.22 points, or 0.04 percent, to 5,262.30.

U.S. stocks ended higher Tuesday, with the Nasdaq hitting a record intraday high, as Wall Street digested a batch of positive earnings reports.

Source : XINHUA