With the economy weakening markedly, banks are flocking to longer-dated government debt on expectations the Bank of Japan will expand its stimulus program. That's creating fresh challenges for BOJ Gov. Haruhiko Kuroda, who's trying to get banks to buy fewer bonds and lend more.

Net purchases by city banks of the nation's sovereign debt maturing in 10 years or longer reached ¥1.16 trillion ($9.7 billion) in August, the highest in 11 months, data from the Japan Securities Dealers Association showed.

The ratio of bank loans to deposits was at 0.673 in August, near a record low touched in May, according to Bloomberg-compiled data based on the central bank's figures.