The Dow Jones Industrial Average added 13.77 points (0.11 percent) in opening trade to 12,652.51.

The broader S&P 500 gained 2.30 points (0.17 percent) at 1,342.50, while the tech-heavy Nasdaq Composite rose 1.72 points (0.06 percent) to 2,829.28.

Earlier Standard and Poor's slashed its rating for Greece, raising the probability of a debt restructuring, as sources said the EU is mulling providing more help to the struggling eurozone member.

The Greek downgrade came amid increasing concern that despite its 110 billion euro ($160 billion) bailout by the IMF and EU last year Greece will not be able to keep on top of it massive debt.