TOKYO, Sept 16 (Reuters) - Japanese government bond yields rose on Thursday after an auction for 20-year notes received tepid demand, while overnight gains in U.S. Treasury yields weighed on sentiment. The 10-year JGB yield rose 1 basis point to 0.040%, while the 20-year JGB yield climbed 0.5 basis point to 0.420%.Earlier in the day, the government auction for 20-year notes received bids worth 3.2 times the amount sold, lower than a bid-cover-ratio of 3.65 times at the previous auction. The 30-year JGB yield also rose 0.5 basis point to 0.650%, but the 40-year JGB yield was flat at 0.720%.Shorter-term note yields also rose, with the two-year JGB yield gaining 0.5 basis point to minus 0.130% and the five-year yield rising 0.5 basis point to minus 0.105%. Benchmark 10-year JGB futures fell 0.08 point to 151.78, with a trading volume of 24,292 lots. (Reporting by Tokyo markets team; Editing by Sherry Jacob-Phillips)