U.S. stock index futures added to earlier gains Tuesday, struggling to recover from the previous session's declines, but investors remained cautious over the uncertainty in the euro zone.

Stocks initially gained at the open Monday, but added to losses throughout the session to finish down more than 1 percent across the board as initial euphoria over Spain's bank bailout fizzled and amid ongoing fears over a global economic slowdown.

European shares were mixed in choppy trade with investors changing focus from Spain to Greece ahead of the June 17 general elections, seen as a referendum on the debt-ridden nation's future in the euro zone.

On the economic front, import prices recorded their biggest decline in almost two years as energy and food costs fell 1.0 percent in May, according to the Labor Department. Meanwhile, export prices slipped 0.4 percent, the first drop since December. Economists had expected a decline of 0.1 percent.

Michael Kors soared after the designer clothing and handbag maker beat profit expectations and handed in a full-year earnings and revenue forecast that exceeded estimates.

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