U.S. stock index futures reversed their earlier gains to turn lower on the final trading day of the month and quarter, after major averages logged a three-day rally.

Futures took a leg lower after Fed Governor Jeremy Stein highlighted the upcoming September policy meeting as a possible time when the central bank may need to consider paring back its QE program, adding that the Fed consider the overall economic improvements since it launched the stimulus instead of giving undue weight to the most recent round of tepid economic data.