By Peter Hoskins
By Samira Hussain & Natalie Sherman
BBC News, New York
By Justin Harper
Us markets have followed Europe and Asia down.
The S&P 500 has fallen 5.21% to 2,397.79, while Dow Jones is down 5.22% at 20,129.70.
The Nasdaq has lost 4.33% to drop to 7,017.38.