Williams told German financial daily Handelsblatt in an interview published on Monday that if U.S. unemployment stayed at 8 percent or rose substantially and growth slowed, then this would raise the question about further monetary measures.

"But I'm today more optimistic about the economy than in January," Williams, a voter this year on the Fed's policy-setting panel, was quoted as saying.

"So far there is no need for further monetary measures," he said, pointing to an improvement in U.S. consumption and available income as well as positive signs in the property market.

The Fed, which is charged with both fighting inflation and fostering maximum employment, has kept short-term interest rates near zero since December 2008 to pull the economy from its worst downturn since the Great Depression.

The central bank has also bought $2.3 trillion in long-term securities and signaled it expects to keep rates low through late 2014 to bolster the recovery further.

Williams had said as recently as February that more bond-buying could well be in the cards.

He said he was very concerned about a renewed intensification of the euro zone's debt crisis. "The worst is not over yet," Williams said, adding that European countries needed to find the right balance between austerity and stimulating growth.

(Reporting by Eva Kuehnen; editing by Patrick Graham)

• A Permanent Economic Emergency