In a letter to House Majority Leader John Boehner, R-Ohio, Lew said Congress "should act as soon as possible to protect America's good credit" before a risk of default becomes imminent.
The borrowing limit will be exhausted by mid-October, Lew said, and the government would be funded after that by "only the cash we have on hand on any given day," which he forecast to be about $50 billion.
Lew noted the U.S. government makes about 80 million payments per month, including Social Security benefits, military salaries and Medicare reimbursements, and said "Congress must act before the middle of October."
Speaking at a press briefing, White House press secretary Jay Carney repeated President Barack Obama's position, saying: "It is unequivocal. We will not negotiate with Republicans in Congress over Congress' responsibility to pay the bills that Congress has racked up, period. It has to pay its bills and it has to vote on a budget, and we hope that Congress fulfills those two basic responsibilities."