Bank of Japan Gov. Haruhiko Kuroda said Friday the central bank will keep its aggressive quantitative easing policy in place until its 2 percent inflation target is achieved, hinting the monetary experiment may take more than two years to work.

"It is not appropriate to say that the monetary easing will only last for two years," although the BOJ took the rare step of committing itself to that very time frame when announcing the step, Kuroda said in a speech in Tokyo.

For the BOJ to affirm its commitment, including to skeptics, "It is appropriate to state that the bank will continue with monetary easing . . . as long as it is necessary" until it achieves the target and maintains it stably, he said. "Making such a commitment will, in the end, further ensure the achievement of the target in two years."