TOKYO, Sept 19 (Reuters) - Japanese government bond prices rose on Thursday, pushing yields down by 2.5 basis points to 3.5 basis points in the 7-year and longer zone, after the U.S. Federal Reserve's decision to delay tapering its bond-buying program took many market participants by surprise.

JGB yields ended the morning session at their intraday lows after fluctuating in a narrow range of 1.5 basis point or less.

As widely expected, the BOJ offered to buy 600 billion yen ($6.06 billion) of JGBs in the 5-year and longer zone under its massive JGB buying program, including 400 billion yen in the 5-year to 10-year zone and another 200 billion yen in the 10-year to 40-year zone.

The BOJ purchased JGBs in the same zones and amounts on Sept 11, one day after a monthly 30-year JGB auction.

The BOJ's outright purchase of JGBs in the secondary market also supported JGBs amid a continued rise in Tokyo stocks.

In the morning session, many regional banks, prefectural cooperatives, and local credit unions sold 9-year to 10-year JGBs on strength, while Tokyo-based life insurers and pension funds bought superlong JGBs amid a lack of aggressive sellers, according to a few money managers and JGB traders.

By early afternoon, the yield on the current 5-year JGBs was down 1 basis point at 0.250 percent, while the yield on 10-year JGBs was down 3 basis points at 0.675 percent, versus 0.690 percent earlier.

The benchmark 10-year JGB yield is still a few basis points above 0.6575 percent, the simple mean between its record low of 0.315 percent on April 5 and its recent high of 1.00 percent on May 23.

In the superlong zone, the yield on the new 20-year bonds (issue number 146) was down 4.5 basis point at 1.605 percent, versus 1.660 percent for the average accepted yield in Wednesday's monthly auction of that maturity.

The 30-year yield was down 3.5 basis points at 1.750 percent.

Lead December JGB futures moved in a 143.92 to 144.11 range and were last up 0.37 point at 144.08.