"Clear and strong commitments" about the future stance of policy are important when official interest rates approach zero and deflation is a fear, Janet Yellen, President of the San Francisco Federal Reserve Bank, said in a report issued on Friday.

Yellen's comment, on how to interpret the lessons learned from Japan's "lost decade of growth," appeared in the bank's latest "Economic Letter."

Yellen also said that it was "unclear" if the program of quantitative easing engaged in by the Bank of Japan from 2001 to 2006 "boosted economic activity much."