The Fed's statement was approved on a 11-1 vote. Jeffery Lacker, president of the Richmond Regional Fed Bank, dissented for the fourth straight meeting. The statement said he opposed the continuation of Operation Twist.
The U.S. economy looks weaker than it did when the Fed last met in April. Growth was more sluggish in the first three months of the year than first estimated.
Job growth averaged only 73,000 in April and May, after average gains of 226,000 per month in the first three months of the year.
The number of people seeking unemployment benefits has risen about 5 percent in the past six weeks, and employers posted sharply fewer job openings in April compared to the previous month.
And economists worry the debt crisis in Europe is worsening, even after Greek election results increased the likelihood that Greece will stay in the euro currency alliance.
Still, U.S. inflation is tame. Core consumer prices, which exclude volatile food and energy costs, have risen just 2.3 percent over the past 12 months. That's close to the Fed's 2 percent target for inflation.
Critics have complained about the Fed's efforts to boost growth over the past three years by purchasing more than $2 trillion in bonds. They say the extra money added to the banking system could ignite inflation once the economy rebounds.
This week's Fed meeting was the first time that the Fed board has been at full strength in six years. Jeremy Stein, a Harvard economics professor, and Jerome Powell, a former private equity executive, attended their first policy meeting since being confirmed by the Senate last month.
AP Economics Writers Paul Wiseman and Christopher S. Rugaber contributed to this report.