"That may look like a shift in the reaction function, but it really isn't," she told the Economic Club of New York. "Global growth was an important factor and we wanted to get ahead of it. And the market response has been favorable."

Yellen added that Fed policymakers "wanted to get ahead of that development and adjust our thinking about the path of policy in order to counteract it before it shows up as a degradation in our forecast for unemployment and inflation."

(Reporting by Jonathan Spicer and Jennifer Ablan; Editing by Chizu Nomiyama)