The U.S. stock market veered sharply lower in midday trading as concerns about global economic weakness intensified, even as Federal Reserve Chair Janet Yellen reiterated her confidence in the U.S. economy.

Financial stocks and energy companies were among the biggest decliners as investors worried that interest rates in the U.S. and elsewhere would remain low and oil prices sank again.

Yellen acknowledged concern about the global economic environment, but said it was too early to tell how big the effect would be on the U.S.

The Dow Jones industrial average dropped 319 points, or 1.8 percent, to 15,595 as of 11:55 a.m. Eastern Time. The Standard & Poor's 500 lost 30 points, or 1.7 percent, to 1,821. The Nasdaq composite fell 49 points, or 1.2 percent, to 4,233.