The Federal Reserve is likely to announce a third round of bond purchases tomorrow, according to almost two-thirds of economists in a Bloomberg survey, while also extending the duration of its zero-interest-rate policy into 2015.

Chairman Ben S. Bernanke and his colleagues on the Federal Open Market Committee will once again roll out unconventional policies to bolster economic growth of less than 2 percent in the second quarter and bring down unemployment stuck above 8 percent for 43 straight months, the survey showed.

"The Fed clearly wants to do more," said Nick Sargen, a former San Francisco Fed economist who oversees $40 billion as chief investment officer at Fort Washington Investment Advisors in Cincinnati. "The economy is looking lackluster, and the Fed has said all along that they feel it's almost immoral that the unemployment rate is as high as it is."