The United States federal government will reach its statutory debt limit of $US16.4 trillion ($A15.85 trillion) - as imposed by Congress - on December 31, Treasury Secretary Timothy Geithner says.
In a letter to the top Democrat in Congress, Geithner said the treasury will shortly begin taking extraordinary measures to "temporarily postpone the date that the United States would otherwise default on its legal obligations".
Geithner said in the letter that the department will take several accounting measures to save approximately $US200 billion beginning next year.
The government borrows about $US100 billion a month, so that would keep the government from reaching the limit for two months.
Geithner said it was harder to predict how long the delay would last because ongoing negotiations over tax and budget policies make it hard to forecast what tax revenue and government spending will be next year.
The debt limit is the amount the government can borrow to help finance its operations.