The S&P 500 fell on Thursday below its 50-day moving average for the first time since November 30, highlighting the benchmark's loss of upward momentum.
The broad index has fallen three of the last four sessions and dropped 1.6 percent over the last two weeks as investors worried about the negative impact of rising oil prices on the global economic recovery.
On Thursday, U.S. stocks tumbled after a rating agency downgrade of Spain rekindled concerns over euro zone debt problems.
The Dow Jones industrial average <.DJI> lost 163.62 points, or 1.34 percent, at 12,049.47. The Standard & Poor's 500 Index <.SPX> fell 18.38 points, or 1.39 percent, at 1,301.64. The Nasdaq Composite Index <.IXIC> dropped 42.95 points, or 1.56 percent, at 2,708.77.