Instead, the Fed said it is "is prepared to take further action as appropriate to promote a stronger economic recovery and sustained improvement in labor market conditions in a context of price stability." It also said that the economy is still expanding "moderately," but that unemployment remained elevate and housing is still "depressed." 

"The Fed [views] the economic expansion as lackluster and has gotten more pessimistic on the prospects for growth," Dan Greenhaus, chief global strategist at BTIG wrote in an e-mail "As a result, they are swapping some securities for others. This is not a convincing response."

The central bank is already pushing hard on the accelerator. The federal funds rate, which is the Fed's main monetary policy tool, is already at an exceptionally low target range between 0% and 0.25%, where it has sat since the financial crisis in 2008. It has also run two quantitative easing programs in which it bought certain assets.