The Bank of Japan will "certainly" reduce short- to medium-term interest rates if it needed to ease monetary policy, Gov. Haruhiko Kuroda said, suggesting that deepening negative rates will be the primary tool to fight growing overseas risks.

Kuroda also said the BOJ already has a flexible framework that allows it to accelerate purchases of exchange-traded funds if markets become volatile, signaling its readiness to moderate stock price falls that could hurt business sentiment.

"On the whole, the world economic outlook ... has become less buoyant. And the timing of (pick-up) of world economic growth has been somewhat delayed," Kuroda said, adding that risks continue to be "fairly high."