WASHINGTON – With Congress and President Obama possibly heading to another showdown over raising the nation's debt ceiling, Treasury Secretary Jack Lew on Sunday pleaded for lawmakers to "take away any cloud of uncertainty about the ability of the United States to pay its bills."

"It's not OK to default," he said on ABC's "This Week."

"I would certainly hope that Congress isn't looking to create confrontations and false crises because we did see, in 2011, how bad that is for the American economy," he said.

He was referring to the partisan standoff, resolved at the last minute, that led Standard & Poor's to downgrade the nation's credit rating for the first time, from AAA to AA+.

A number of congressional Republicans want to impose conditions for raising the government's $16.7 trillion debt limit. Some have insisted on deeper spending cuts, while others want concessions from the president on other policy issues.

"If we're going to raise the debt limit, then we've got to do something about what's causing us to spend more money than what we bring in," House Speaker John Boehner said on CNBC last month.

"So guess what? We're going to have a debate, and we're going to have a negotiation."

But Lew signaled that the administration isn't willing to negotiate.

The government can make it through Labor Day before needing to raise the debt limit as the Treasury Department uses "extraordinary measures" to pay its bills, Lew said on CNN's "State of the Union."

The Congressional Budget Office has said the debt limit will not need to be raised until October or November.

"Congress shouldn't wait until the last minute," Lew said. "They should just raise the debt limit and take away any cloud of uncertainty about the ability of the United States to pay its bills."

The battle over the debt limit is heating up as Republicans are preparing for a separate fight among themselves and with the Obama administration over the budget.

Some GOP lawmakers are threatening to vote against funding the government beyond Sept. 30 unless money for Obama's health care law is eliminated. That stance would risk a government shutdown. Other Republicans worry that a repeat of the 1995 Clinton-era government shutdown would hurt the party.

Lew cautioned congressional Republicans against seeking to slash domestic programs in order to prop up defense.

"To be clear, what the president said — and has written to Congress — is that they cannot fix the problems created by the across-the-board cuts — known as sequestration — by cutting domestic priorities in order to fund defense," Lew said on NBC's "Meet the Press." "That's unacceptable."

Sen. Mike Lee, R-Utah, appearing on "Fox News Sunday," said, "We all know the government is going to get funded. The only question is whether the government gets funded with Obamacare or without."