U.S. stocks were flat on Monday, as global markets continue to weigh the U.S. government shutdown and looming debt ceiling crisis.

In company news, shares of Alcoa Inc. (NYSE: AA) fell 0.94 percent to $7.89 in morning trading on Monday after Morgan Stanley cut its rating on the former Dow component to "equal weight" from "overweight" just as the aluminum producer is scheduled to report earnings after the close on Tuesday.

Shares of Apple Inc. (NASDAQ: AAPL) edged higher after investment firm Jefferies Group LLC (NYSE: JEF) upgraded the tech giant's stock to "buy" from "hold." In addition, the firm lifted Apple’s stock price target to $600, from $425. 

On Monday, Apple rose 1.77 percent to $491.57 per share in morning trading.

BlackBerry Limited (BBRY) shares rose 3.64 percent to 7.97 after a report said the smartphone maker is in talks to sell parts of itself to Google Inc. (NASDAQ: GOOG), Cisco Systems Inc. (NASDAQ: CSCO), and SAP, according to Reuters. The possible bids come as BlackBerry agreed recently to be acquired by major shareholder Fairfax Financial in a deal worth $4.7 billion.

The Dow Jones industrial average fell 84.78 points or 0.56 percent, to 14,987.80. The S&P 500 was down 7.48 points or 0.44 percent, to 1,682.98. The Nasdaq Composite dropped 16.43 points or 0.43 percent, to 3,791.10.