At midday, the Dow Jones Industrial Average decreased 106.24 points, or 0.37 percent, to 28,539.02. The S&P 500 was down 10.98 points, or 0.34 percent, to 3,229.04. The Nasdaq Composite Index erased 43.56 points, or 0.48 percent, to 8,963.05.
Most of the Dow component companies traded in red territory, with IBM dropping 1.28 percent, leading the laggards.
Nine of the 11 primary S&P 500 sectors declined, with communication services down 0.70 percent, lagging behind the rest groups.
U.S. equities have enjoyed a strong rally in December, with the main indexes hitting record highs last week amid year-end optimism.
Market sentiment has been lifted by easing tensions over U.S.-China trade relations after the two countries agreed on the text of a phase-one trade deal earlier this month.
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