TOKYO, Dec 20 (Reuters) - Japan"s 10-year government bond yields fell on Monday on higher demand for save-haven debt as fears over an economic slowdown loomed amid the swift spread of the Omicron variant worldwide. The 10-year JGB yield fell 0.5 basis point to 0.040%. Benchmark 10-year JGB futures rose 0.13 point to 152.24, with a trading volume of 9,575 lots. The spread of Omicron saw the Netherlands go into lockdown on Sunday and put pressure on others to follow, though the United States seemed set to remain open. The fears dented risk appetite, sending the Nikkei stock average (.N225) down more than 2% earlier the session. Yields on longer-end notes were flat, with the 20-year JGB yield at 0.445% and the 30-year JGB yield at 0.650%. The 40-year JGB yield was also flat at 0.695%. The two-year bonds were not traded and their yield remained at minus 0.115%. The five-year yield fell 0.5 basis point to minus 0.105%. Reporting by Tokyo markets team; Editing by Sherry Jacob-Phillips