U.S. stock-index futures signaled a lower open on Wednesday after data had private-sector job creation in May failing to live up to expectations. The ADP private sector payrolls report, usually a warm up act for the official payrolls, found the private sector added 179,000 jobs last month versus a 215,000 estimate.

Read MoreJob creation in private sector disappoints The Commerce Department reported the trade gap increased to $47.2 billion in April. And, other data from the Labor Department found workers less productive in the first quarter, revised down by 3.2 percent. Markets "typically don't respond to either of these figures and will wait more eagerly for the ISM services index at 10 a.m." Eastern, noted Peter Boockvar, chief market analyst at the Lindsey Group.

In particular, traders are holding fire until Friday's U.S. government's nonfarm payrolls report. Analysts polled by Reuters forecast 218,000 were created in May, below April's 288,000 figure.

In addition, markets await any stimulus measures the European Central Bank might announce at Thursday's Governing Council meeting. Interest rate cuts, cheap loans to banks or even Federal Reserve-style asset purchases could be on the cards and traders are positioning themselves for any eventualities. "With no one in a rush to do anything, markets are still hovering near their recent highs, but with no one willing to bet big ahead of the central bank meetings and payrolls, the natural position squaring is leading to a consolidation of price action," analysts at brokerage Capital Spreads said in a morning note. Meanwhile, G-7 leaders will meet in Brussels on Wednesday with the agenda set to be dominated by the situation in Ukraine, relations with Russia as well as other foreign policy topics. The meeting, which was originally scheduled to take place in Russia, will conclude Thursday afternoon.