U.S. stock index futures pointed to a lower open on Thursday after weaker-than-expected retail sales and the European Central Bank left monetary policy unchanged, disappointing investors who hoped the ECB would prevent a debt crisis from engulfing the euro zone.

U.S. retailers, including discounter Costco Wholesale Corp to Gap Inc , posted disappointing April sales, and the early report suggested hopes for a consumer rebound may be getting ahead of the pace of recovery.

Tim Ghriskey, chief investment officer of Solaris Asset Management in Bedford Hills, New York, said the retail sales were disappointing. While economic data on the consumer has improved, and, anecdotally, we see a lot of people in stores, it doesn't look like in April there was much (spending), he said. It calls into question the strength of consumer and willingness to step up and spend.

The government said the number of U.S. workers filing new applications for unemployment insurance fell slightly less than expected last week, implying a measured job market recovery. Also, the Labor Department said non-farm productivity rose at a 3.6 percent annual rate, the smallest advance in a year, after expanding at a brisk 6.3 percent pace in the fourth quarter.

S&P 500 futures fell 7.3 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 dropped 46 points, while Nasdaq 100 futures dipped 17.5 points.

Eleven out of 17 retailers posted same-store sales fell short of expectations, according to a tally by Thomson Reuters.

U.S. non-farm productivity growth slowed sharply in the first quarter, government data showed on Thursday, suggesting businesses will have to raise employment to boost output.

The ECB's decision to scrap collateral rating requirements on Greek debt was an acknowledgment of the extent of the country's economic recovery program, ECB President Jean-Claude Trichet said. He dismissed out of hand the prospect of Greece defaulting on its debt.

The futures sold off maybe in disappointment that they didn't get more from (ECB President) Trichet but again, they shouldn't have expected it today, said Peter Boockvar, equity strategist at Miller Tabak + Co.

(Reporting by Angela Moon; editing by Jeffrey Benkoe)