Doubts over Chinese yuan put BoJ in a bind

China's devaluation of the yuan exposes an undefended flank in the Bank of Japan's efforts to jolt its flagging economy out of decades of deflation, which rely heavily on a solid pick-up in overseas demand.

A growing number of Japan's central bankers are privately voicing concern that the problems behind China's currency move will hit Asian demand harder than expected, threatening a Japanese export rebound they hope will stave off the need for another splurge of monetary easing.

Since April 2013 the bank has ploughed 170 trillion yen ($1.37 trillion) into a radical quantitative easing programme that some, including some policymakers on the bank's board, think has gone far enough - particularly given its questionable returns.