From CNBC:

Geithner said more than $500 billion in Treasuries will mature in August, the month in which the Treasury has said it will no longer be able to pay all the nation's bills without an increase in the $14.3 trillion debt cap.

"There is no guarantee that investors would continue to re-invest in new Treasury securities" if the United States defaults on non-debt obligations, Geithner wrote. "In fact, some market participants have already indicated that they would be disinclined to do so."
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For those who are interested, the Secretary's letter can be found at:

Also, in its concluding statement from its annual Article IV consultation with the U.S., the IMF declared, "the federal debt ceiling should be raised expeditiously to avoid a severe shock to the economy and world financial markets." In terms of downside risks facing the U.S., the IMF also noted:

Unfavorable fiscal outcomes. These could take the form of a sudden increase in interest rates and/or sovereign downgrade if an agreement on consolidation does not materialize or the debt ceiling is not raised soon enough. These risks would also have significant global repercussions, given the central role of U.S. Treasury bonds in world financial markets.