US stocks tumble again as sell-off deepens; Dow Jones falls over 500 points

US stocks tumbled anew on Thursday in another trading session with big swings, as investors remained on edge after several days of volatile trading.

Major indexes fell about 2 per cent in late afternoon trading.

The benchmark S&P 500 was set for a second day of declines, following sharp swings in recent sessions including its biggest drop in more than six years that pulled equities away from record highs.

"The dust hasn't settled yet, and I think both buyers and sellers are trying to figure out what this market really wants to do," said Jonathan Corpina, senior managing partner for Meridian Equity Partners in New York.