Several Bank of Japan board members said the central bank must be vigilant about the risk of inflation accelerating more than expected, minutes of the March policy meeting showed on Monday.

A few of the nine-member board also said they saw some "positive signs" emerging in Japan that suggested the economy was making progress toward achieving the BOJ's 2% target, the minutes of the March 9-10 meeting showed.

The board debated how companies were continuing to hike prices to pass on rising raw material costs, and price increases broadening to services, the minutes showed.

"It was important to use a wide range of data and look back on the basic mechanism behind price moves, to deepen our understanding on inflation developments," one member said.

At the March meeting, the BOJ maintained its ultraloose policy, including a 0.5% cap for the 10-year bond yield that had come under attack from markets betting on a near-term interest rate hike in the wake of recent rises in inflation.

While some saw positive signs emerging on the price front, many members said there was "extremely high" uncertainty over Japan's economic outlook that warranted keeping monetary policy ultraloose, the minutes showed.

"The BOJ must focus on the risk of missing the chance of achieving its price target with a premature policy shift, rather than that of being too late in modifying policy," one member was quoted as saying.

Another member said any debate over a policy shift must be made cautiously, as a reversal of ultraloose policy would have wide-ranging effects on the public, the minutes showed.

The March meeting was the final one chaired by Haruhiko Kuroda, who retired as governor in April and was succeeded by Kazuo Ueda.

Markets are rife with speculation that Ueda will steer the BOJ away from the radical stimulus measures deployed by Kuroda, which are drawing increasing criticism for distorting market pricing and crushing financial institutions' profits.