U.S. stock index futures extended losses on Monday after Standard & Poor's credit ratings agency cut its long-term outlook for the United States to negative from stable.

S&P 500 futures fell 16.40 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 fell 139 points and Nasdaq 100 futures fell 34.25 points.

(Reporting by Angela Moon, Editing by Kenneth Barry)