Wall Street was set for a flat open on Wednesday as investors awaited clarity on media reports that the European Central Bank would, with broad support from its council members, unveil an unlimited, sterilized program of bond purchases.

U.S. stock index futures had been lower earlier on concerns over global economic growth after data suggested weakness in the euro zone's private sector, and economic bellwether FedEx cut its profit forecast. But they sharply trimmed losses on the ECB news and then seesawed between gains and losses.

"The market did get a bounce earlier reacting to that (ECB news). Basically it's good news in the sense that it should help ease some of the fears of the crisis expanding," said Peter Cardillo, chief market economist at Rockwell Global Capital in New York.

"It doesn't cure all of the euro's problems, obviously. But I'd say it's a major step in the right direction to stabilize the euro."

S&P 500 futures were off 1.8 points and were in line with fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures were flat, and Nasdaq 100 futures were off 4 points.

FedEx Corp , the world's No. 2 package delivery company, cut its profit outlook for the current quarter, saying weakness in the global economy was hurting demand for overnight international shipments more than anticipated. The announcement was made after the closing bell on Tuesday. The stock was off 2.9 percent to $85.01 in premarket trade.

Nokia and Microsoft Corp will take the wraps off the struggling European company's most powerful smartphone on Wednesday, in what may be their last major shot at winning back a market dominated by Apple , Samsung <005930.KS> and Google .

Facebook Inc promised not to sell stock to cover a nearly $2 billion tax bill and said it will allow employees to cash in their stock weeks ahead of schedule, moving to soothe nervous investors and its own staff as its share price spirals downward from its $38 IPO price. Facebook shares were up 2.7 percent at $18.21 in premarket trade.

Data showed U.S. nonfarm productivity increased at a much faster clip than previously thought in the second quarter as businesses squeezed more output from employees. But market reaction was muted.

(Additional reporting by Wanfeng Zhou; Editing by Dave Zimmerman)