U.S. stock index futures signal a mixed opening Tuesday as optimism following the Greek election results fades and Spain's increasing borrowing costs raise investor concerns.

Futures for the Dow Jones Industrial Average rose 0.05 percent, futures for Standard & Poor's 500 index fell 0.14 percent and futures for the Nasdaq 100 index remained flat.

Investor confidence is being affected by the awareness that the Greek election results mark no end of the country's economic problems.

The borrowing costs of Spain and Italy are increasing, adding to the euro zone woes. Spain is under huge debt pressure after its 10-year bond yield rose over 7 percent. Investors are also worried that the contagion will spread to Italy, which is faced with mounting debt pressures.

The U.S. Census Bureau's housing starts, measuring the change in the annualized number of residential buildings that began construction, will be reported Tuesday. It is expected to rise to 720,000 in May, up from 717,000 in April.

On Monday, the U.S. markets were mixed. The Dow Jones Industrial Average was down 0.2 percent, the S&P 500 Index rose 0.1 percent and the Nasdaq Composite Index was up 0.8 percent.

Major European indices were in the green, but investors remained watchful as talks are going on among political parties to form a coalition government in Greece. London's FTSE 100 rose 38.19 points, Germany's DAX 30 index advanced 27.48 points and France's CAC 40 moved up by 2.23 points.

Asian markets fell Tuesday following global cues as investor sentiments turned negative amid the revival of euro zone concerns.