U.S. stock index futures fell on Thursday after the European Central Bank left monetary policy unchanged, disappointing investors looking to the ECB to prevent a debt crisis from engulfing the euro zone.

The decision was expected, but some investors were disappointed the central bank took no action as confidence in the euro has been badly shaken on fears the euro-zone debt crisis would spread.

The euro extended losses to hit a new 14-month low against the dollar on Thursday following the rate decision.

Peter Cardillo, chief market economist at Avalon Partners in New York said the decision to keep the monetary policy unchanged was expected but with the euro collapsing, investors who wanted to see some sort of a defensive movement by the ECB.

S&P 500 futures fell 3.2 points and slightly 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 dropped 21 points, while Nasdaq 100 futures dipped 4.5 points.

The S&P 500 has fallen 3 percent for the week as euro-zone concerns triggered a broad sell-off.

Investors will focus on weekly first-time claims for jobless benefits due from the Labor Department at 8:30 a.m. EDT Economists in a Reuters survey forecast a total of 440,000 new claims, compared with 448,000 in the prior week.

The Labor Department also releases preliminary first-quarter productivity and unit labor cost data at 8:30 a.m. EDT. Economists look for a rise of 2.5 percent in productivity versus a 6.9 percent increase in the prior period. Unit labor costs are expected to fall 0.7 percent compared with a 5.9 decline in the previous report.

Kraft Foods Inc is due to report quarterly results, and analysts expect earnings of 45 cents per share, unchanged from a year ago, according to Thomson Reuters I/B/E/S.

U.S. stocks sagged Wednesday as violent protests erupted in Greece and fears grew that the Greek debt crisis could spread to other European economies. The euro's sharp drop pushed investors into safe havens like U.S. Treasurys and the dollar.

(Reporting by Angela Moon; editing by Jeffrey Benkoe)