profit warning by UnitedHealth (UNH.N) led to a 5.65 percent drop in its stock, making the health insurer the biggest drag on the Dow Jones industrial average and the S&P 500. It also sent the shares of competitors Anthem (ANTM.N) and Aetna (AET.N) down more than 6 percent each.

The S&P healthcare sector .SPXHC was the worst performer among the 10 major S&P sectors with a 1.63 percent decline.

Adding to the pain in healthcare, drugmaker Pfizer (PFE.N) fell 3.06 percent after reports that its talks to buy Allergan (AGN.N) and redomicile in Ireland were in final stages. Allergan lost 2.81 percent.

Intel Corp (INTC.O) jumped 3.44 percent after boosting its annual dividend. The chipmaker and Apple (AAPL.O), up 1.27 percent, added more upward pressure to the S&P 500 than any other stocks.

Mobile payments company Square (SQ.N) soared 45 percent in its highly anticipated market debut, while dating website operator Match Group (MTCH.O) popped 23 percent on its first trading day.