Some Bank of Japan policymakers warned against premature debate about exiting from its ultraloose policy despite their concern about the rising cost of prolonged easing, according to minutes of the BOJ's Policy Board meeting in March, reinforcing yet again the challenge of hitting the nation's elusive inflation target with a diminishing toolkit.

The nine-member board took time debating at the March meeting how to best communicate their policy intentions. Some said growing market interest about when the BOJ could whittle down its massive stimulus program was partly behind the recent market volatility, the minutes show.

"It was important for the BOJ to thoroughly explain to the public ... that the economy had not yet reached a phase where it should consider the timing and measures of a so-called exit from monetary easing," some members said.