Stock index futures fell on Tuesday, weighed by worries over weaker-than-expected import data from China and a cautious mood ahead of the Federal Reserve's meeting on monetary policy.

Speculation has been growing that the Fed will send a clear signal it is prepared to print more money to support a faltering U.S. economic recovery, if necessary.

While more easy money could encourage investors to buy stocks, however, a more cautious forecast from the Fed, whose outlook has still been for a moderate recovery, may heighten concerns that the economic growth may be losing its steam.

A mere acknowledgment of a blip might also disappoint investors who have been betting the Fed would make a more concrete move, such as buying bonds to pull down market rates.

Investors increasingly believed that in a crisis or downturn, the Fed would step in and inject liquidity until the problem got better. Invariably, the Fed did so each time, said Peter Boockvar, equity strategist at Miller Tabak & Co in New York.

The FOMC has thus put themselves in the position today that if they don't follow through, we will see a sharp sell-off in Treasuries, MBS (mortgage-backed securities), corporates (bonds) and stocks as markets have been so trained to get bailed out by the Fed every time economic data turns down.

A much slower-than-expected growth in July imports in China fueled a sharp decline in European and Asian markets as it further pointed to slowing domestic demand at the world's third-largest economy.

S&P 500 futures fell 8.1 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 dipped 57 points, and Nasdaq 100 futures fell 11.25 points.

The dollar rose against major currencies on Tuesday as traders trimmed short positions ahead of the outcome of the Fed meeting, while oil futures fell 1.6 percent to $80.14 a barrel.

Economic data on tap includes the second-quarter preliminary reading for labor costs and productivity at 8:30 a.m. (1230 GMT) and June wholesale trade at 10 a.m. (1400 GMT). Analysts in a Reuters survey forecast a 1.3 percent growth in labor costs and 0.2 percent for productivity.

Companies expected to report quarterly results on Tuesday include Walt Disney Co .

Technology shares will be in the spotlight after top contract chipmaker TSMC <2330.TW> posted record monthly sales for a fourth month on strong demand, while its sales and profit could peak in the current quarter.

Industrial conglomerate Eaton Corp expects to achieve its 2010 China sales target of $1 billion, its chief executive said on Tuesday.

Exxon Mobil Corp has formally ended a deal with state-funded Qatar Petroleum to develop a new $6 billion petrochemicals facility in Ras Laffan, according to Middle East Economic Digest.

Shares of Novell Inc sank 2.7 percent to $5.85 in premarket trading after the company cut its third-quarter revenue outlook.

U.S. stocks rose on Monday but in a quiet session as investors were wary of taking new positions before the Fed's statement.

(Editing by Padraic Cassidy)