In a letter addressed to congressional leaders, Geithner said Treasury can take extraordinary measures to stretch the nation's checkbook by $200 billion, a move that normally would give the government two months leeway but because of the uncertainty about the "fiscal cliff," that would not be the case this time.
Geithner urged lawmakers to act to end the "significant uncertainty" in the nation's finances to prevent "default on ... legal obligations."
Geithner also warned the uncertainty could delay the filing of tax returns, exacerbating the problem.
Lawmakers and President Barack Obama are scheduled to return to Washington Thursday and resume negotiations to avert the "fiscal cliff," the effects of which will begin to be felt Jan. 1 unless a deal is reached.
Notable deaths of 2014 [PHOTOS]