In a letter to House Speaker John Boehner, R-Ohio, Lew said the Treasury was implementing extraordinary measures to pay the government's bills as of Friday, with the expiration of a temporary suspension of the statutory debt limit that was part of the Continuing Appropriations Act of 2014 -- a compromise spending bill that led to the end to a government shutdown.
"Because Congress has not acted to approve normal borrowing authority, Treasury must begin implementing extraordinary measures that enable us, on a temporary basis, to protect the full faith and credit of the United States and to continue paying the nation's bills," he wrote.
"As I have noted previously, these measures are more limited than in past debt limit impasses," Lew wrote.
The secretary, who notified Congress last month the department will likely exhaust extraordinary measures in late February, said Friday he is "not confident" extraordinary measures will last beyond Feb. 27.
"At that point, Treasury would be left with only the cash on hand and any incoming revenue to meet our country's commitments," he said.
Lew estimated the government will have about $50 billion cash on hand at that point, but he said "any foreseeable cash balance would be exhausted quickly."
"If Treasury has insufficient cash on hand, it would be impossible for our nation to meet all of its obligations for the first time in history," he said.
"Protecting the full faith and credit of the United States is the responsibility of Congress because only Congress can extend the nation's borrowing authority," Lew said.
"As such, I respectfully urge Congress to move as quickly as possible, raise the debt limit, and provide certainty to the economy and to the financial markets."