The United States will reach its $16.4 trillion borrowing limit on Dec. 31 and undertake “extraordinary measures” to avoid default, the Treasury Department informed congressional leaders in a letter on Wednesday.

The notice about the debt ceiling came packaged with a stark warning from Treasury Secretary Timothy Geithner, who said the policy uncertainty stemming from the “fiscal cliff” makes it impossible for him to say with certainty how much time lawmakers have left to raise the borrowing cap.

“Given the significant uncertainty that now exists with regard to unresolved tax and spending policies for 2013, it is not possible to predict the effective duration of these measures,” Geithner wrote.