The Bank of Japan has begun shifting its focus from supporting growth to phasing out its massive stimulus, taking the first tentative steps toward a potentially momentous move for the world economy.

Current and former central bankers familiar with internal discussions say an informal debate is underway on how to prepare for an exit from the BOJ's 13-month-old "quantitative and qualitative monetary easing."

The stimulus is a centerpiece of Prime Minister Shinzo Abe's campaign to end two decades of deflation and fitful growth, and BOJ Governor Haruhiko Kuroda has vowed to keep cheap cash flowing until his 2 percent inflation target is in plain sight.