U.S. stocks fell sharply in volatile trading Monday, extending a steep sell-off from the previous session.

The S&P 500 pulled back 3.4 percent to break below 2,700. It also erased its gains for the year.

The broad index had traded positive earlier on Monday as the tech sector briefly rose. The S&P 500 also traded down more than 5 percent from an all-time high set last month and broke below its 50-day moving average, a key technical level.

"Breaking the early lows of the day means the correction could go on for longer," said Art Cashin, UBS director of floor operations.... Read More: CNBC