Uncle Sam may not have enough cash to make interest payments on his debt a month from now if Congress does not approve an increase on the federal borrowing limit, currently at $18.1 trillion.

Such inaction similar to what happened two years ago, traders fear, could roil financial markets, rattle investor confidence in U.S. Treasuries as a safehaven investment and make it face a possible downgrade on its credit rating.

On Thursday, Treasury Secretary Jack Lew urged U.S. lawmakers in a letter to raise the legal debt ceiling because the government would not be able to borrow more money after Nov. 3, two days earlier than what he told them last week.

Without borrowing capability, the Treasury's cash on hand would fall below $30 billion in two weeks, which would be less than the expected interest payments due on Nov. 16, analysts said.

"The coupon interest payment of $33.8 billion due on November 16 to holders of marketable securities has to be viewed as vulnerable," Stone & McCarthy Research Associates said in a research note.

Given the unpredictability of daily receipts and payments, the Treasury's cash holdings might be exhausted even sooner.

"There is a possibility that the cash balance could be exhausted even before November 10 without an increase in the limit," Goldman Sachs economists wrote in a note.