TOKYO, June 28 (Reuters) - Japanese government bond yields edged higher on Monday tracking U.S. Treasury yields, while markets shrugged off the Bank of Japan’s weak bond-purchase operation. Yields on 20-year JGBs added half a basis point at 0.430% midway through the afternoon session, while 30-year bond yields gained 1 basis point at 0.685%. Five-year paper yielded minus 0.100%, down half a basis point, while two-year yields were flat at minus 0.115%. The new 10-year note was untraded, last yielding 0.045%. Benchmark 10-year JGB futures fell 0.06 point to 151.66. The benchmark 10-year Treasury yield was little changed in Asia at 1.5275%, holding above 1.5% after posting its biggest weekly gain since early February on Friday. Even so, Katsutoshi Inadome, a strategist at Mitsubishi UFJ Morgan Stanley Securities, sees bonds largely stuck in the absence of new catalysts following the hawkish shift at the U.S. Federal Reserve earlier this month. “Sideways trading will probably continue,” Inadome said. The BOJ offered to buy 450 billion yen ($4.07 billion) each of three- to five-year debt, and five- to 10-year securities, along with 50 billion yen worth of bonds with 25-year or greater maturities. The results for the longer-dated paper were on the weak side, while the operation for shorter-dated notes went smoothly, according to a person at a domestic securities house. ($1 = 110.6400 yen) Reporting by Tokyo markets team; Editing by Amy Caren Daniel Our Standards: The Thomson Reuters Trust Principles.