U.S. Treasury yields hit session highs on Thursday after a German newspaper reported Greece's aid program would be extended.
The yield on the 10-year note rose to a session high of 2.37 percent, above its Wednesday close of about 2.31 percent. Thirty-year bond yields hit a high of 3.16 percent, up 6 basis points on the day.
Greece's creditors plan to offer to extend its existing aid program until the end of this year, but without the participation of the International Monetary Fund, Germany's Die Zeit newspaper reported in its online edition on Thursday.
However, the yields cooled a bit after EU Vice President for the Euro Valdis Dombrovskis denied the report. The 10-year was last quoted at 2.33 percent, up 2 basis points. The 30-year bond rose 3 basis points to 3.13 percent.
Read MoreMarkets on inflation watch after Fed
Earlier, U.S. Treasury prices rose as investors continued to digest the Federal Reserve's statement. The Fed on Wednesday left interest rates unchanged and lowered its forecasts for growth and the federal funds rate. Separately, the Philadelphia Federal Reserve Bank index rose to 15.2 in June, beating expectations.
Government bond prices were also boosted by jitters over Greece.
Euro zone finance ministers meet in Luxembourg on Thursday to discuss a reforms-for-aid deal for the cash-strapped country, but expectations of a deal are low.
Greece is set to default on a 1.5 billion euro ($1.7 billion) debt repayment to the International Monetary Fund on June 30 unless it receives fresh funding from the bodies which oversee its bailout—the European Union, the International Monetary Fund and the European Central Bank.
On the data front, May consumer prices had their largest increase in over two years while weekly jobless claims data came in at 267,000, below the expected 275,000. In addition, the Philadelphia Federal Reserve Bank index rose to 15.2 in June, beating expectations.
"In her press conference Yellen was pressed on a number of factors including Greece and the strength of the U.S. dollar and she did accept that both could well continue to act as a drag on the U.S. economy, which would appear to suggest that unless we see a significant improvement in the data in the next few months a move in September remains unlikely at this point," said Michael Hewson, chief market analyst at CMC Markets, in a note.
—Reuters contributed to this report.