This year's expected debt-ceiling fight is different from the 2011 and 2013 debt-ceiling crises, when the Treasury Department was able to use emergency borrowing authority for several months, Lew told Washington's Bipartisan Policy Center.
He won't be able to do that this time because of a surge in February spending -- mainly due to the annual rush of people seeking tax refunds this month, he said.
The payments the Treasury Department has to pay this month run into the billions of dollars, he said.
"Given these realities, it is imperative that Congress move right away to increase our borrowing authority," Lew told the center.
His department anticipates it will exhaust all "extraordinary measures" by the end of this short month, he said.
"It is imperative that Congress move right away to increase our borrowing authority," he added. "It would be a mistake to wait until the 11th hour to get this done."
Congress temporarily suspended the debt limit in October as part of a deal to reopen the government after a 16-day shutdown.
The limit goes back into force Friday.
U.S. indebtedness was about $17.3 trillion Monday.
President Obama has said he will not negotiate over the debt limit, calling it a congressional responsibility, not a bargaining chip.
House Speaker John Boehner, R-Ohio, said last week he wouldn't let the nation default on its debt.
But some House Republicans have said they would agree to increase the national debt only if the administration agrees to spending cuts or changes to the Affordable Care Act.