In China, the Shanghai stock index plunged 8.5 percent Monday, its worst fall since 2007, despite government efforts to calm the market.
Teva Pharmaceutical Industries of Israel jumped 8 percent after saying it would buy Allergan's generic drug business. Allergan jumped 7.5 percent.
The Dow Jones industrial average lost 133 points, or 0.8 percent, to 17,435 as of 9:35 a.m. Eastern time.
The Standard & Poor's 500 index lost 13 points, or 0.6 percent, to 2,066. The Nasdaq composite declined 47 points, or 1 percent, to 5,039.
Bond prices rose. The yield on the 10-year Treasury note fell to 2.22 percent.