The US will hit its debt ceiling by 17 October, leaving the government with half the money needed to pay its bills, the Treasury Secretary has warned.
Jack Lew said that unless the US is allowed to extend its borrowing limit, the country will be left with about $30bn to meet its commitments.
"Net expenditures on certain days can be as high as $60bn," he said.
The US government and Republicans are at stalemate over extending the credit limit needed to avoid default.
President Barack Obama and the Democrats have said they will not negotiate with Republicans over their demand that the government agrees budget cuts in return for backing a rise in the borrowing limit.
Mr Lew's comments underline how close Washington is to running out of money. Failure to reach a deal would be "catastrophic" for the US economy, he said in a letter to House Speaker John Boehner.
"Treasury now estimates that extraordinary measures will be exhausted no later than October 17. We estimate that, at that point, Treasury would have only approximately $30bn to meet our country's commitments.
"If we have insufficient cash on hand, it would be impossible for the United States of America to meet all of its obligations for the first time in our history," Mr Lew said.
