U.S. stock index futures signaled a further decline on Wall Street on Friday as concerns about the impact of losses related to U.S. subprime mortgages stoked flight from global equity markets.

Even as major central banks moved to calm the jittery markets by injecting additional liquidity into the financial system, equity markets headed lower across Asia and in Europe, boosting the safety allure of government bonds.

The concerns we have in the credit markets aren't domestic, they are global, said Art Hogan, chief market analyst, Jefferies & Co. in Boston.

The futures look pretty soft and I think a lot of that is being driven by news that came out of Countrywide, talking about the spread or the contagion of this credit tightening.

On Thursday, U.S. stocks suffered their second worst decline of the year, with the Dow Jones industrial average sinking nearly 400 points, or 2.83 percent, while the Standard & Poor's 500 Index slid almost 3 percent.

After the close of trade signs of further turmoil in the subprime mortgage market emerged from a regulatory filing from the nation's largest mortgage lender Countrywide Financial Corp, which said it faces unprecedented disruptions in the debt market and secondary market for mortgages.

S&P 500 futures were down 14.2 points, below fair value, a mathematical formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures were off 112 points, and Nasdaq 100 futures shed 15.5 points.

Countrywide shares were down more than 14 percent in Europe.