U.S. stocks edged up on Wednesday, a day before a crucial meeting of the European Central Bank, on media reports that policymakers will unveil a bond-buying plan to bring down crippling borrowing costs in debt-troubled euro zone economies.

Optimism about the ECB was tempered by FedEx Corp , the world's No. 2 package delivery company, which cut its profit outlook for the current quarter late on Tuesday, citing weakness in the global economy. The stock fell 1.4 percent to $86.

The ECB is ready to waive seniority status on government bonds it buys under a new program which it is set to agree on at Thursday's Governing Council meeting, central bank sources told Reuters on Wednesday.

Bloomberg earlier reported that the ECB would, with broad support from its council members, unveil an unlimited, sterilized program of bond purchases. The ECB has been expected to be cautious about disclosing the size of its bond-buying, given opposition from Germany's central bank.

Further details of the plan will be revealed by ECB President Mario Draghi after Thursday's meeting, but some analysts cautioned the ECB may opt to wait until after the German constitutional court rules on the region's bailout funds on September 12 to announce any new steps.

"There's a little bit of speculation right now on whether President Draghi actually announces something tomorrow or whether they wait to see what the court ruling is next week and then wait to announce it then," said Ryan Larson, head of equity trading at RBC Global Asset Management in Chicago.

"The expectation is that ultimately some form of additional stimulus will be announced, probably more so in the case of the ECB than the Fed right now. That's probably what continues to somewhat backstop the market from any meaningful losses for the time being."

The Dow Jones industrial average <.DJI> gained 23.51 points, or 0.18 percent, to 13,059.45. The Standard & Poor's 500 Index <.SPX> rose 0.66 points, or 0.05 percent, to 1,405.60. The Nasdaq Composite Index <.IXIC> added 2.20 points, or 0.07 percent, to 3,077.26.

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 .

Shares of Facebook Inc rebounded from an all-time low after the company 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. The stock was up 0.9 percent at $18.64.

Equities have received a boost in recent months on expectations the ECB would start buying Spanish and Italian government bonds to ease the pressure on those countries' bond markets and that the Federal Reserve will adopt new stimulus to prop up the economy.

"We think there's more risk than reward to U.S. equities right now given...the economy is continuing to decelerate as well as the markets putting a lot of faith in central bankers that we think is misguided," said Gary Flam, portfolio manager at Bel Air Investment Advisors in Los Angeles.

(Additional reporting by Angela Moon, editing by Dave Zimmerman)