The BOJ cut its buying of Japanese government bonds with 10 to 25 years to maturity by 20 billion yen ($188.8 million) to 160 billion yen, from 180 billion yen in previous operations since July.

The move came as the 10-year Japanese government bond yields fell below minus 0.200%.

The BOJ has said it roughly defines its policy target of "around zero percent" for the 10-year yield as 20 basis points above or below zero percent.

The cut in JGB purchase on Friday is considered to be a technical move to keep the 10-year yields in line with the current target.

Still reducing asset purchase at a time when many central banks in the world are easing their monetary policies highlights the BOJ's lack of policy ammunition after building up a massive balance sheet and cutting interest rates to negative levels.

Benchmark 10-year JGB futures rose 0.32 point to a record close of 154.73, extending gains even after the BOJ's operation.

Although U.S. Treasuries fell on Thursday as calm returned to U.S. equity markets, many investors are flocking to the safety of bonds as they see no clear end to U.S.-China trade woes, which intensified further this week.

The 10-year JGB yield fell 3 bps to minus 0.225%, a three-year low. On the week, it was down 5 bps, the biggest fall since December.

The 20-year JGB yield fell 3.5 bps to 0.095%, while the 30-year JGB yield fell 4 bps to 0.220%. For the week, they were down 8.5 and 10 bps respectively, both the biggest drop in about three years.

At the shorter end of the market, the five-year JGB yield  fell 2 bps to a three-year low of minus 0.305%, while the two-year JGB yield fell 3 bps to a 2-1/2-year low of minus 0.275%.