U.S. stock index futures extended gains on Thursday after the a surprise rate cut by the European Central Bank.

The ECB lowered its main interest rate by 25 basis points to 1.25 percent as the euro zone's worsening debt crisis outweighed the concern over persistently high inflation.

S&P 500 futures rose 15.8 points and were above fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration of the contract. Dow Jones industrial average futures gained 124 points, and Nasdaq 100 futures rose 17 points.

(Reporting by Angela Moon; editing by Jeffrey Benkoe)