Stocks tumbled amid concerns that events surrounding Ukraine are escalating and that China's slowing growth will rub off on the global economy and bite into corporate profits.
U.S. Secretary of State John Kerry said a "serious series of steps" would be imposed by the U.S. and Europe on Monday if the referendum in Crimea takes place on Sunday as planned. Russia meanwhile launched new military exercises near its border and reports circulated of shots being fired at aircraft.
"We don't have panic selling here," said one stock trader. "We're getting a lot of panic questions."