Federal Reserve officials decided to keep the central bank’s balance sheet at a record to spur the slowing economy after completing $600 billion of bond purchases this month.

“The economic recovery appears to be proceeding at a moderate pace, though somewhat more slowly than the committee had expected,” Fed Chairman Ben S. Bernanke said at a press conference after a meeting of the Federal Open Market Committee. Bernanke and his colleagues on the panel cut their growth forecasts for this year and next and raised their estimates for the unemployment rate, driving stocks lower.

Bernanke, in his second post-meeting press conference, didn’t specify when the central bank might end its “extended period” of interest rates near zero. At the same time, he stressed that the economy has improved since last August, when he first indicated that the Fed might embark on a second round of large-scale asset purchases to stave off the threat of a broad-based decline in prices.