Sources say Japan's central bank has purchased a record amount of long-term government bonds for a single day.

The action on Thursday was aimed at curbing rising yields.

The Bank of Japan bought about 4.6 trillion yen's worth of bonds during the day.

The move came after investors sold the instruments, sending yields higher. They were responding to market speculation that the BOJ may further adjust monetary policy.

The central bank tweaked policy last month, allowing long-term interest rates to fluctuate in a wider band.

Selling bonds usually raises their yields.

The yield on the benchmark 10-year Japanese government bond hit the BOJ's newly set upper limit of half a percent for four consecutive trading days.

The BOJ is scheduled to hold its monetary policy meeting on Tuesday and Wednesday next week.

A question mark hangs over what measures the bank will take amid the market speculation.