U.S. stocks dropped on Friday as concerns about slowing economic growth persisted, causing investors to sell shares of bellwether companies such as plane maker Boeing Co. and putting all three major indexes on course to end the week lower.

Recent weakness in prices of commodities such as copper, which are seen as indicators of economic growth, added to negative sentiment on the economic outlook, traders said.

The Standard & Poor's 500 index was headed for its biggest weekly drop since July. The Dow Jones industrials and the Nasdaq, meanwhile, were set to snap a two-week winning streak.

"My sense is that people are concerned about a slowing economy and that's why you're seeing this decline in the market," said Stephen Massocca, co-chief executive at San Francisco-based investment bank Pacific Growth Equities.

The Dow Jones industrial average was down 70.42 points, or 0.57 percent, at 12,350.83. The Standard & Poor's 500 Index was down 6.88 points, or 0.49 percent, at 1,411.42. The Nasdaq Composite Index was down 12.96 points, or 0.54 percent, at 2,402.89.

Boeing shares were among those leading declines in thin trading, with many traders heading out for the Christmas long weekend. Boeing dropped 1.6 percent to $88.50.

The stock was the biggest drag on the Dow. Shares of chemical maker DuPont dropped 1.3 percent to $48.36.

Before the bell, the Commerce Department reported that core prices, excluding volatile food and energy costs, were flat in November after rising 0.2 percent in October, bolstering expectations that the Federal Reserve could have room to cut interest rates next year.

The report followed a Thursday report that showed the U.S. economy in the third quarter grew at a slower rate than earlier estimated, sparking concern that economic growth may be slowing faster than expected.