U.S. stock index futures dipped on Thursday as nagging concerns about Greece's debt woes undercut a solid quarterly earnings report and stronger forecast from economic bellwether United Parcel Service Inc.

UPS climbed 4.8 percent to $68.60 in premarket trade after the package delivery company reported quarterly earnings that topped estimates and raised its forecast for the year.

Earnings continue to be impressive. It's hard not to look at the earnings cycle, even though we are coming off admittedly low levels, with admittedly easy comparables. You have to start somewhere, said Barry Ritholtz, director of equity research at Fusion IQ in New York. Things from an earnings perspective look much better than they have in quite a long time.

Worries about Greek debt problems persisted, pushing the euro lower against the dollar and driving up Greek bond yields as investors sought clarity into whether it will need to tap a planned euro zone aid package.

S&P 500 futures slipped 2.6 points and were below 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 lost 20 points, and Nasdaq 100 futures shed 5 points.

Investors may be ready to take some profits after U.S. stocks notched a fifth straight day of gains on Wednesday as strong corporate results and March retail sales pushed the S&P 500 past 1,200 for the first time in 18 months.

MGM Mirage slumped 5.3 percent to $14.60 premarket, a day after the casino operator projected a wider-than-expected quarterly loss due to weak Las Vegas results, including its new CityCenter development.

Other companies set to report results on Thursday include Google Inc , Advanced Micro Devices Inc , PPG Industries Inc
and People's United Financial Inc
.

Wall Street also waited for a string of economic data,

including March industrial output, the New York Fed's Empire State manufacturing index, weekly jobless claims, the Philadelphia Fed business index for April, and the April NAHB housing market index.

European stocks edged higher on Thursday, with drugmakers rising after Roche Holding AG said sales beat estimates, though gains were capped by weaker mining stocks.

Asian stocks rose after China's economic growth accelerated and bellwether U.S. technology and financial companies reported string results, boosting appetite for riskier assets.

(Reporting by Chuck Mikolajczak; editing by Jeffrey Benkoe)