Washington, D.C. ? As recently as two months ago, the Federal Reserve sounded optimistic about the economic recovery. Now the central bank is taking a new step that shows it is clearly more worried, but economists say it probably won’t help much.

The Fed said Tuesday that it would spend a relatively small amount of money — about $10 billion a month, economists estimate — buying government debt. The move is designed to drive interest rates on mortgages and corporate borrowing at least a little lower and help the economy grow faster.

In a statement after a one-day meeting, the Fed said the pace of the recovery “has slowed in recent months.” After its last meeting in late June, the Fed was rosier, saying that the recovery was “proceeding” and the job market actually improving.

The decision to buy government debt, using proceeds from Fed investments in mortgage bonds, was a shift from earlier this year, when the Fed was laying out plans to roll back some of the measures it took during the financial crisis.