U.S. stock index futures extended losses on Tuesday after Citigroup Inc announced fourth quarter results.

S&P 500 futures fell 4.2 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 dipped 40 points, and Nasdaq 100 futures lost 3.25 points.

Shares of Citigroup fell 1.5 percent to $3.37 in premarket trade.

(Reporting by Angela Moon)