U.S. Treasury tumbled on Monday after the European Central Bank said it would maintain emergency assistance for Greek banks, but adjusted haircuts on collateral.
Yields inched toward session lows after the report after paring losses after a report showed U.S. service sector growth inched higher in June.
The Institute for Supply Management said its services sector index increased to 56.0, just short of analysts' expectations of 56.2 in a Reuters survey, from 55.7 in May. A reading above 50 indicates expansion in the sector.
Prices rallied earlier, pushing yields lower, after a surprise "no" vote in a Greek referendum on the country's bailout terms fueled fears of Greece exiting the euro zone and boosted demand for safe-haven bonds.
The yield on the benchmark 10-year Treasury fell about 9 basis points to about 2.29 percent—its lowest level in over two weeks. The 30-year bond yield was down 10 basis points at about 3.08 percent.
The "risk off" mood in global markets following Sunday's Greek referendum also lifted German Bund prices, but dealt a blow to stocks with European markets lower and U.S. stock futures also trading in the red.
"Global markets did not take it (the Greek vote) well… the risk off trade sets the market tone for now," Bill Blain, a strategist at Mint Partners in London, said in a note.