U.S. stocks were flat Tuesday as investors were expecting policy makers to come up with some measures to keep the euro zone intact.

Investors were cautious as finance ministers and central bank governors of the Group of Seven leading industrialized nations hold an emergency conference call Tuesday to discuss the European debt crisis, trying to figure out some way to prevent the problems from further worsening.

On the economic news front, the Institute for Supply Management reported that U.S. service sector grew at a slightly faster pace in May, marking the 29th straight month of expansion.

Investors were also awaiting Fed's report later in the week, hoping the U.S. central bank would give some hint about the third round of quantitative easing policy (QE3).

"Don't forget, the beige book will be released on Wednesday, and will surely be looked at even more closely than usual by traders as we inch toward that 20 June FOMC policy announcement," said Stephen Guilfoyle, an economist at Meridian Equity Partners. "Will we see coordinated action by the planet's central banks? Chances are that the time is not yet upon us, but I won't rule it out until Thursday,"he added.

In midday trading, the Dow Jones industrial average dipped 1.17 points, or 0.01 percent, to 12,100.29. The Standard & Poor's 500 added 2.03 points, or 0.16 percent, to 1,280.21. The Nasdaq Composite Index rose 2.77 points, or 0.10 percent, to 2,762.78.