U.S. stock index futures fell on Thursday after rating agencies said they may downgrade Greece's sovereign debt, reigniting concerns over possible defaults in the euro zone that have dogged markets recently.

Moody's said Thursday a change in its rating would depend on whether Athens smoothly enacted fiscal reform plans, while Standard & Poor's said late Wednesday a downgrade of one or two notches in the next month remains possible.

The sovereign debt concerns offset euphoria in the last session, when stocks rallied after U.S. Federal Reserve Chairman Ben Bernanke said interest rates would remain low for an extended period. Bernanke resumes his testimony on Capitol Hill today.

S&P 500 futures fell 5.1 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 15 points, and Nasdaq 100 futures were down 9.25 points.

On the economic front, weekly jobless data, due at 8:30 a.m. EST (1330 GMT), is expected to show initial claims for jobless insurance fell to 455,000 from 473,000 in the previous week.

Durable goods for January are due at the same time, with economists in a Reuters survey expecting a 1.5 percent rise in orders versus a 1.0 percent gain in December. The numbers will be another measure of the strength of the economy.

Weak housing and consumer sentiment data earlier this week dampened investor confidence in an economic recovery.

Quarterly earnings are expected from H.J. Heinz Co , Dr Pepper Snapple Group Inc , Gap Inc and Safeway Inc .

In Asia, Japan's Nikkei average <.N225> fell 1 percent on Thursday as the yen rose broadly. In Europe, the pan-European FTSEurofirst 300 <.FTEU3> index of top shares fell 0.3 percent higher.

(Reporting by Edward Krudy; editing by Jeffrey Benkoe)

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