A top Federal Reserve official said on Friday she could see the U.S. central bank raising interest rates in the first half of the year, earlier than some expect, given that the economy is on a "very firm footing."

"The Fed is preparing the public and the markets," Cleveland Fed President Loretta Mester said on the Fox Business Network television channel. "I do believe that inflation will gradually move back to our target, so I could imagine interest rates going up in the first half of the year."

The Fed's key rate has been near zero since late 2008. Investors and economists generally expect a hike around mid-2015 - with many eyeing policy meetings set for June, July and September - and comments from influential Fed officials appear to back this.

Monetary tightening will depend on the economic data and how close inflation and unemployment are to its goals, said Mester, a centrist who does not have a vote on policy this year.

As "the real rate of return on capital goes up, we want to make sure that our short term policy rate is moving up with it," she added. "There is no predetermined formula."

Fed policy meetings are set for mid-June, late July and mid-September.