Federal Reserve Chair Janet Yellen said interest rates will probably be raised in 2015 and made the case for a cautious approach to subsequent increases that will keep borrowing costs low for years to come.

In a speech in San Francisco on Friday, she emphasized that the coming tightening cycle will be unlike any other in recent times, as the U.S. economy continues to heal from the worst recession since the Great Depression.

"The actual path of policy will evolve as economic conditions evolve, and policy tightening could speed up, slow down, pause, or even reverse course depending on actual and expected developments in real activity and inflation," she told a conference hosted by the San Francisco Fed.