TOKYO, May 28 (IFR) - Japanese government bond prices were steady on Thursday, underpinned by the Bank of Japan's purchases under its massive asset buying programme.

Besides the central bank's buying, cash activity was limited to transactions among dealers, traders said.

The BOJ offered to buy 170 billion yen ($1.67 billion) in JGBs maturing beyond 10 years in addition to 250 billion yen in the 3-year to 5-year zone.

The BOJ did not reduce its purchase amount in the superlong zone as some investors had expected. Some believe the central bank will buy less in that zone as it seeks to shorten the average duration of its JGB holdings from around 8 years to its target of 6 to 7 years.

The yields on the current 10-year and 20-year JGBs were unchanged from Wednesday's levels at 0.590 percent and 1.445 percent, respectively, while the 30-year yield dipped half a basis point to 1.685 percent.

Ten-year lead June JGB futures moved in a 145.24-145.30 range in the morning session before finishing at midday up 0.06 point at 145.29. ($1 = 102.0700 Japanese Yen) (Reporting by Masatsugu Hisatsune; Editing by Shri Navaratnam)