Government bond prices dipped slightly in light trading Monday.
Earlier in the day, Treasurys rose after Standard & Poor's warned that it was becoming more likely that Greece would default on its debt.
The rating agency cut Greece's credit rating to CCC on Monday. S&P said the risk of Greece defaulting on its debts within the next year has "increased significantly." The rating agency doubts that Greece will be able to access bond markets to finance its budgets in 2012.
The news helped briefly lift prices for Treasurys, which are used as a refuge by many global investors.
Bond prices fell later in the day. Treasury prices have been rising over the past two months on fears that Europe's debt crisis could spread and on signs that the U.S. economy was slowing down. Those worries pushed bond yields to 2011 lows last week.
Many bond traders expect the trend to reverse and for bond yields to rise in the coming months.
The price of the 10-year Treasury note slipped 9.3 cents for every $100 invested. The dip in price pushed the yield to 2.99 percent on Monday, down from 2.97 percent late Friday. When bond prices fall their yields rise.
The 10-year yield touched 2.94 percent last week, its low for the year.
In other trading, the 30-year bond dropped 34.3 cents, while its yield rose to 4.20 percent from 4.18 percent. The yield on the two-year note was unchanged at 0.40 percent.
The three-month Treasury bill paid a 0.03 percent yield at a discount of 0.04 percent.