Washington -- U.S. Treasury Secretary Timothy Geithner asked Congress to increase the $12.1 trillion debt limit on Friday, saying it is "critically important" that they act in the next two months.

Mr. Geithner, in a letter to U.S. lawmakers, said that the Treasury projects that the current debt limit could be reached as early mid-October. Increasing the limit is important to instilling confidence in global investors, Mr. Geithner said.

The...