The Bank of Japan on Thursday offered to buy an unlimited amount of Japanese government bonds at fixed rates for the first time since the introduction of a new policy framework-a sign of its concerns over recent rises in yields.

The move is the first clear sign from the central bank that it intends to take action to keep a lid on rising yields, and took market participants by surprise.

"I thought there was still a lot more room left" before the BOJ took action, said Masahiro Ichikawa, senior strategist at Sumitomo Mitsui Asset Management.