Most Wall Street economists expect the Federal Reserve will undertake another major economic stimulus program, even though the U.S. central bank did not announce any new action at its policy meeting on Tuesday, according to a Reuters poll.

Ten of 17 primary dealers, the large financial institutions that do business directly with the Fed, said they expect the central bank to undertake a new program of buying mortgage-backed securities. That compares with 13 of 20 primary dealers calling for a new MBS purchase program in a similar poll conducted on December 2.

Four more dealers said another program of MBS buying was possible. Four of the 10 dealers who expected further MBS purchases said the Fed would also embark on a new program of buying Treasury debt.

The poll was conducted on Tuesday after the Federal Reserve reiterated it plans to hold interest rates steady at the current ultra-low level near zero through at least mid-2013.

The Fed also pointed to turmoil in Europe as a big risk to the U.S. economy, leaving the door open to a further easing of monetary policy even as it noted some improvement in the U.S. labor market.

Seventeen of the 21 primary dealers responded to Tuesday's poll.

Five of the 17 dealers said they expect the Fed to further ease monetary policy through a new program of Treasury debt purchases, while three more dealers said such purchases were possible.

The Fed has already been through two rounds of quantitative easing. Under these programs, known as QE1 and QE2, the purchases of MBS and Treasury debt totaled about $2.3 trillion.

The Fed's current stimulus plan, dubbed Operation Twist, is a $400 billion program that extends the maturity of the central bank's Treasuries holdings in a bid to lower mortgage rates and other long-term borrowing costs. The program is scheduled to last through the end of June 2012.

Some analysts believe that before the Fed embarks on any new Treasury or MBS purchase programs it will try to ease monetary policy by changing the way it communicates with the financial markets.

It is important to keep this (quantitative easing) option on the table and to be prepared in the event conditions melt down once again, but the first option and likely very soon is the use of greater transparency to support financial conditions, said Robert Diclemente, chief economist at Citigroup in New York.

Asset purchases will come second if necessary later, he said.

(Additional reporting by Pam Niimi; Editing by Chizu Nomiyama)

(This story corrects to 4 possible MBS buyers in 3rd paragraph, corrects Dec 2 data in table)