U.S. stock index futures rose on Monday as Wall Street looked to extend three weeks of gains as some investors said shares could rally in a thinly traded, holiday-shortened week.

Mergers and acquisitions picked up, a sign that some companies see value in the market. AT&T Inc said it would acquire wireless spectrum licenses from Qualcomm Inc in a deal worth $1.9 billion.

Stocks have rallied strongly this month, with the S&P 500 index up more that 5 percent in December to a two-year high.

Although investor participation may be limited, the outlook is for a strong finish into year-end, said Andre Bakhos, director of market analytics at Lek Securities in New York. Any news that would move the market lower would have to be of a high magnitude.

S&P 500 futures rose 5 points and were 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 37 points, while Nasdaq 100 futures fell 1 points.

U.S. markets will be closed on Friday to observe the Christmas Day holiday the following day.

EarthLink Inc said it will acquire One Communications Corp for $370 million, and Raytheon Co will acquire Applied Signal Technology Inc for $490 million. Applied Signal shares rose 7.7 percent to $37.70 in premarket trade.

Underscoring expectations of strong holiday spending, U.S. online sales are up 12 percent to $27.5 billion so far this season compared with a year ago, according to research firm comScore. For details, see [ID:nN19285412]

Adobe Systems Inc reports quarterly results later in the day. Analysts expect the software maker to earn 52 cents per share, up from 39 cents a year ago.

Genzyme Corp , fighting off a hostile $18.5 billion bid from Sanofi-Aventis SA , will take another stab at persuading investors its experimental multiple sclerosis drug is worth more than Sanofi's projections.

Food and beverage company Sara Lee Corp has been in talks to sell itself to Brazilian meat producer JBS , but the two companies are at odds over price, a source said.

In European equity markets, the FTSEurofirst 300 <.FTEU3> rose 1 percent in early trade, with energy shares supported by firmer crude prices as cold weather in Europe and parts of the United States boosted demand for heating fuel.

(Editing by Padraic Cassidy)