Worsening tensions in Ukraine are sending markets lower, overshadowing an encouraging report on U.S. economic growth.
Ukraine's president declared that Russian forces have entered his country and called an emergency meeting of the nation's security council.
The Dow Jones industrial average fell 64 points, or 0.4 percent, to 17,056 as of 9:35 a.m. Thursday. The Dow has risen for the previous three days and it still higher for the week.
The Standard & Poor's 500 index fell seven points, or 0.4 percent, to 1,992.
The Nasdaq composite fell 17 points, or 0.4 percent, to 4,552.
European markets also fell. Germany's DAX lost 1.4 percent and France's CAC-40 fell 0.7 percent.
Bond prices rose as investors sought out lower-risk assets. The yield on the 10-year Treasury note fell to 2.32 percent.