Investors, however, are worried that rating agency Standard & Poor's will strip the United States of its top-notch rating soon, raising the world's biggest economy's borrowing costs.

The Treasury auctioned $23 billion in one-month bills due Sept.1 at an interest rate of 0.070 percent, matching the level at the one-month auction on May 8. For more see

Last week, it auctioned one-month bills at a rate of 0.06 percent

Copyright Reuters, 2011