It's up to Congress, not the president, to decide how large of an increase in the debt ceiling is needed, a top Obama economic adviser said Monday morning.

That statement fills in a key detail regarding President Obama's debt limit strategy, which he has insisted will be to refuse to negotiate with congressional Republicans.

By leaving the amount of the increase to Congress, Obama could avoid directly asking lawmakers to raise the limit, thereby denying Republicans a chance to open up talks.

Appearing at a press breakfast organized by Politico, National Economic Council Director Gene Sperling said that "it is the responsibility of Congress to decide how long or how often" they want to raise the debt ceiling.

Asked by ABC's Jonathan Karl if the administration would be open to a brief, week-long increase to avoid a brush with government default, Sperling responded that "longer is better in terms of economic certainty and jobs" but that it would be up to Congress.

Treasury Secretary Jack Lew has said that he will exhaust his options to create headroom under the debt ceiling by Oct. 17, and that beyond that point he will only have cash on hand and incoming revenues with which to pay the government's bills. The public debt outstanding reached the limit in May.

House Republicans, who are in a separate legislative battle over funding the government, have not coalesced around a strategy for passing a bill to raise the limit in the lower chamber. On Sunday, House Speaker John Boehner ruled out a clean debt ceiling hike without concessions from the president.

Sperling pushed back against Boehner just as hard Monday morning, insisting that Obama refuses to negotiate with Republicans not just for political reasons, but also out of fear for America's long-term well-being.

"If you sanction through negotiations the legitimacy of someone threatening default, that's going to happen again," Sperling claimed, warning that in the long run such negotiations would "increase the chances that we eventually default" in future showdowns over the debt ceiling.

Sperling appeared at the breakfast with Jason Furman, chairman of Obama's Council of Economic Advisers. Furman said Obama is willing to help Boehner protect himself from the right wing of his caucus, offering the speaker a "talking point" to give to conservatives — but not one that would amount to a concession. "We're not negotiating with him," Furman said.

Furman noted that in February Boehner was able to get a suspension of the debt ceiling passed in the House without significant trouble. It meant that the debt ceiling hike was an economic "non-event," and showed that Boehner "has the power to do that" and avoid the possibility of a disaster resulting from the Treasury missing a payment on an obligation.