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.

Moody's said a change in Greece's rating would depend on whether Athens smoothly enacted fiscal reform plans, while Standard & Poor's said a downgrade by one or two notches in the next month remains possible. That could increase borrowing costs and exacerbate Greece's problems.

I've been hearing that people are fearing the worst, which would be that this is similar to the (1997) Asian crisis, said Kim Caughey, senior equity research analyst at Fort Pitt Capital Group in Pittsburgh. Is Greece starting the downward cascade?

Caughey said that on the upside, European nations would likely bail out Greece in a worst-case scenario.

A busy political calendar may divert investor attention from fundamental issues. Federal Reserve Chairman Ben Bernanke will resume his testimony on Capitol Hill, while U.S. President Barack Obama will launch a last-ditch bid to salvage his stalled healthcare overhaul.

S&P 500 futures fell 6.6 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 55 points, and Nasdaq 100 futures were down 10 points.

The S&P 500 is up more than 40 percent from its 12-year low on March 9, but down nearly 4 percent from a 15-month peak reached last month.

In company news, Coca-Cola Co said it will buy the North American operations of one of its largest bottlers, Coca-Cola Enterprises Inc , in a largely cashless deal. Coke shares fell 2.7 percent to $53.65, while Coca-Cola Enterprises jumped 29 percent to $24.78.

Shares in pharmacy benefit manager Express Scripts Inc rose 9.4 percent to $96 after it reported better-than-expected fourth-quarter profit late Wednesday and forecast 2010 earnings above estimates.

Weak housing and consumer sentiment data earlier this week dampened investor confidence in an economic recovery. Data later today may shed more light on the economy's prospects.

Investors will get weekly jobless data, due at 8:30 a.m. EST (1330 GMT), which is expected to fall to 455,000 from 473,000 in the previous week, according to a Reuters survey.

At the same time, durable goods for January, another measure of the economy's strength, are expected to show a 1.5 percent rise in orders versus a 1.0 percent gain in December, a Reuters survey showed.

(Reporting by Edward Krudy; editing by Jeffrey Benkoe)