Stock index futures trimmed early gains on Thursday, tracking European markets lower, as speculation swirled that France's sovereign debt rating may be downgraded.

The FTSEurofirst 300 <.FTEU3> index of top European shares fell from earlier highs and was up 0.2 percent.

Futures initially pointed to a higher open after Spain sold all of its debt at an auction and after solid earnings from Bank of America and Morgan Stanley.

Traders could not pinpoint the source of the rumors early in the New York session, but the drop in futures highlighted continued sensitivity to the region's debt crisis.

German Bund futures hit a record high of 140.78, up 42 ticks on the day. Italian, Spanish and French bond yield spreads over benchmark German Bunds widened.

Spain, the latest trouble spot in the euro zone debt crisis, sold 2.5 billion euros ($3.3 billion) of bonds, but yields rose as Madrid struggled to tame its deficit.

The market is relieved at the fact Spain was able to fund itself in a pretty strong showing. Demand was strong even though they had to pay for higher prices, said Peter Cardillo, chief market economist at Rockwell Global Capital in New York.

Bank of America Corp , the No. 2 U.S. bank, climbed 4.3 percent to $9.30 in premarket trade and Morgan Stanley jumped 4.6 percent to $18.48 after reporting results.

DuPont and Co's quarterly profit and sales beat expectations, helped by price hikes and strong sales of herbicides and genetically modified seeds.

According to Thomson Reuters data, 35 companies in the S&P 500 are due to report Thursday. Notables after the close include Microsoft Corp , Chipotle Mexican Grill Inc and SanDisk Corp .

Of the 56 S&P 500 companies reporting through Wednesday morning, 79 percent beat estimates.

S&P 500 futures rose 4.4 points and were slightly above 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 gained 33 points, and Nasdaq 100 futures advanced 14.5 points.

On the economic front, clues on the health of the labor market could come with the release of weekly initial jobless claims at 8:30 a.m. EDT (1230 GMT). Economists forecast a total of 370,000 new filings, compared with 380,000 in the prior week.

Due at 10 a.m. EDT (1400 GMT) are the Philadelphia Federal Reserve Bank's April business activity index, the Conference Board's leading economic indicators for March and existing home sales for March from the National Association of Realtors.

Biotechnology company Human Genome Sciences Inc surged 101.3 percent to $14.43 premarket after it rejected an unsolicited $2.6 billion bid from long-time partner GlaxoSmithKline Plc .

CVR Energy Inc climbed 7.7 percent to $30.02 premarket after the crude oil refiner said it reached a deal with Carl Icahn for a $2.26 billion tender offer by the investor to go forward.

Asian shares traded in tight ranges before the Spanish bond auction amid renewed concerns over the euro zone's debt crisis.

(Reporting By Chuck Mikolajczak; editing by Jeffrey Benkoe)