Wall Street opened lower Friday as fresh warnings about Greece kept investors away from risky assets while trading was expected to be volatile and light throughout the day heading into a long weekend.
Belgian deputy Prime Minister Didier Reynders issued a new warning over Greece, saying it would be a "grave professional error" if central banks and companies were not preparing for a Greek exit from the euro zone.
Earlier, U.S. stock index futures rose on speculation that European authorities could soon initiate new aid measures. Credit Agricole said it expected the ECB to make a move at its next rate-setting meeting on June 6 and announce new measures such as another round of emergency funding for the region's banks.
Trading was expected to be volatile with light volume as traders stay away from making new bets or positions heading into the Memorial Day holiday weekend. U.S. markets will be closed on Monday.
In economic news, U.S. consumer sentiment rose to its highest level in more than four years in May as Americans stayed optimistic about the job market, while higher income households expected to see bigger wage increases, a survey showed.
The Thomson Reuters/University of Michigan's final reading on the overall index on consumer sentiment rose to 79.3 from 76.4 in April, topping forecasts for 77.8 and an initial May reading of the same.
It was the highest level since October 2007.
Morgan Stanley will adjust thousands of trades to ensure outstanding limit orders to sell will be filled at no more than $42.99 a share for Facebook stock from last Friday's botched initial public offering, the firm told its brokers on Thursday, according to several who listened to the call.
Green Mountain Coffee Roasters Inc independent director Douglas Daft resigned from the board this week, at a time when concerns about slowing growth and allegations of accounting malpractice have hammered the stock.
Reuters contributed to this report.