(Corrects fourth graph to say $1.7 trillion instead of billion)

A top Federal Reserve official said on Friday he believes the U.S. economy is no longer at risk of slipping into a damaging deflationary spiral but that it would be unwise for the Fed to suddenly shift course.

The official, Philadelphia Federal Reserve President Charles Plosser, told the Wall Street Journal in an interview that with deflation less of a risk, the only justification for the Fed's controversial $600 billion bond buying program is high unemployment.

The Fed cut short-term interest rates to near zero in December of 2008 to pull the economy out of a deep recession caused by one of the worst financial crises in U.S. history.

The Fed then bought $1.7 trillion of longer-term securities, in an effort to continue to try to support economic growth. When the fledgling recovery faltered in the middle of last year, the Fed launched another bond-buying program.

One of the justifications for the program was that inflation was so low, the United States risked slipping into a damaging deflationary spiral.

Plosser, one of the most conservative on the Fed, has been a skeptic of the latest round of securities purchases.

He said on Friday he doesn't believe Fed policies can bring down unemployment rates, which eased to 9 percent in January from 9.4 percent the previous month but are still well above the 5 percent to 6 percent the Fed considers full employment.

Monetary policy can't retrain people. Monetary policy can't fix those problems, he said.

Plosser is a voting member of the Fed's policy setting panel this year, but backed January's decision to keep the bond buying program going.

Explaining his decision, he said credibility demands that the bank not stomp on the brakes and then floor the accelerator.

Why do you want to signal something and then yank it out from under the market? That's just not a good way to conduct policy.

(Writing by Mark Felsenthal; Editing by Jackie Frank)