One of the most dovish U.S. central bankers on Tuesday said the economy remains vulnerable the longer inflation remains too low, and he again warned that policy stimulus should be removed "only gradually."

Eric Rosengren, president of the Federal Reserve Bank of Boston, was the only policymaker to formally dissent against the Fed’s decision last month to trim its bond-buying program. While he has long warned that high unemployment can permanently scar the labor market, on Tuesday he outlined the parallel threat posed by low prices.

"Inflation rates persistently below the stated target can be a cause for real concern," Rosengren said in remarks prepared for delivery to a Connecticut business and industry association.

"At very low inflation rates, a sizable negative shock to the economy can result in negative inflation – deflation – which can become entrenched in expectations, leading to a protracted period of deflation."

The Fed reduced to $75 billion, from $85 billion, the amount of assets it buys each month in an aggressive effort to spur investment and hiring in the protracted wake of the Great Recession. Fed Chairman Ben Bernanke has said that the program, known as quantitative easing, will likely be wound down throughout this year as the U.S. economy improves.

Rosengren, who will not have a vote on policy this year under the Fed’s rotating system, is to the dovish side of the spectrum of Fed policymakers. The doves have generally held sway since the recession, but that may now be changing since unemployment hit a five-year low of 7 percent in November and as concerns grow about pumping so much money into the economy.

Rosengren acknowledged that economic conditions are improving, and he expects gross domestic product growth of about 3 percent this year. Yet he questioned whether temporary factors were keeping inflation near 1 percent, below the Fed’s 2 percent goal.

He said it was both "striking" and concerning that inflation remains so low well into the slow recovery from recession, especially given all the monetary stimulus from the Fed.

The Fed "continues to miss both elements of its dual mandate from Congress — inflation and employment — by fairly large margins," Rosengren said in his remarks.

"As the economy continues to improve, we should reduce and ultimately remove the unusual support" the Fed has provided, he said. "But this support should be removed only gradually."