Lew warned in the new letter that Congress will likely need to raise the debt ceiling by late February to avoid a potential default on the country's obligations. 

As part of the bill to reopen the government in mid-October, the debt limit was suspended through Feb. 7. The Treasury can use so-called "extraordinary measures" after that point to keep borrowing and paying the nation's bills for a few weeks thereafter.

"While this forecast is subject to inherent variability, we do not foresee any reasonable scenario in which the extraordinary measures would last for an extended period of time," Lew wrote in the letter, which was addressed to House Speaker John Boehner (R-Ohio), House Minority Leader Nancy Pelosi (D-Calif.), Senate Majority Leader Harry Reid (D-Nev.), and Senate Minority Leader Mitch McConnell (R-Ky.).

In the letter, Lew urged Congress to raise the debt ceiling before Feb. 7, and "certainly before late February."