The Fed's June policy-setting meeting takes place about a week before Britain votes on a referendum to exit the European Union, an event nicknamed Brexit.

"We’re going to have a lot better information by the middle of June about Brexit and what polls show and how the markets are reacting," Williams said on the sidelines of a conference on monetary policy at Stanford University's Hoover Institution. "Clearly if there’s an expectation that it actually will pass and the markets will react to that then we have to take that into consideration in terms of how it affects the U.S. economy and the outlook."

(Reporting by Ann Saphir; Editing by Chizu Nomiyama)