U.S. stock index futures point to a mixed opening Wednesday ahead of economists' expectations that the Fed will announce an extension of its 'Operation Twist'  monetary policy designed to boost the economy. 

Futures on the Dow Jones Industrial Average fell 0.18 percent, futures on the Standard & Poor's 500 index rose 0.08 percent and futures on the Nasdaq 100 index rose 0.27 percent.

Investors are likely to focus on the Federal Open Market Committee's (FOMC) decision on interest rates. The FOMC statement, which is the primary tool the panel uses to communicate with investors about its monetary policy, will be closely watched.

Many economists expect the Fed to announce an extension of its Operation Twist stimulus program. This program is due to expire in less than two weeks, but some say the U.S. central bank could decide to extend it at a policy meeting that ends on Wednesday, according to Reuters. 

Operation Twist involves the Fed buying $400 billion in longer-dated Treasuries in the open market, with these purchases funded by selling comparable amounts from its holdings of short-dated government debt, Reuters said.

Hope for additional stimulus measures at the FOMC meeting have intensified among market players. Investors are hoping that the debt crisis looming over the euro zone and the faltering U.S. economy will push the Fed to take these stimulus measures.

Extending Operation Twist buys time for the Fed to evaluate incoming data. It has the advantage of not expanding the Fed's balance sheet, Credit Agricole said in a note.

On Tuesday, U.S. markets were up as investor sentiment was positive following increasing hopes that the Fed will extend Operation Twist, which many hope will restore economic growth. The Dow Jones industrial average was up 0.75 percent, the S&P 500 Index rose 0.98 percent and the Nasdaq Composite Index was up 1.19 percent.

Major European indices were in the red as investors remained watchful as talks among Greek political parties to form a coalition government in Greece continued. London's FTSE 100 fell by 12.76 points, Germany's DAX 30 index declined 23.43 points and France's CAC 40 moved down by 8.90 points.

On a positive note, the yield on ten-year government bonds in Spain edged a little lower Tuesday. However, the trend may not be sustained in the days ahead, underlining the fact that the banking bailout for Spain will not address the country's broader fiscal problems.

Asian markets rose Wednesday following global cues as investor sentiment turned positive amid expectations that central banks globally will announce coordinated stimulus measures to boost the global economy.