U.S. President Barack Obama speaks about the economy to mark the five-year anniversary of the U.S. financial crisis at the White House in Washington September 16, 2013. REUTERS/Kevin Lamarque

By Mark Felsenthal and Roberta Rampton

WASHINGTON (Reuters) - President Barack Obama appealed to business leaders on Wednesday to urge Congress to approve an increase in the U.S. debt limit without any conditions attached and avoid a default that is possible as early as mid-October.

"It is going to be important for all of you over the next several weeks to understand what's at stake and to make sure that you are using your influence in whatever way we can to get back to what used to be called regular order around here," Obama told the Business Roundtable.

The president's speech to about 100 top corporate executives was part of an effort to focus on domestic budget and economic issues after a month dominated by foreign policy, mainly the conflict in Syria.

"I'm happy to negotiate with them around the budget," Obama said. "What I will not do is to create a habit, a pattern, whereby the full faith and credit of the United States becomes a bargaining chip to set policy."

He reiterated his vow not to negotiate over raising the debt limit, which Republicans want to tie to spending cuts or to blocking his signature healthcare program. He said he was willing to haggle over the terms of the budget, but not against the threat of a government shutdown or killing the healthcare program.

The U.S. Treasury is expected to exhaust measures to avoid exceeding the $16.7 trillion debt limit as soon as mid-October. If the borrowing cap is not raised, the United States will not be able to pay all of its bills and would go into default.

At the same time, the healthcare program, formally called the Affordable Care Act but popularly called Obamacare, faces a deadline October 1 when sign up for insurance exchanges opens to the public. The six-month sign-up period is deemed critical to the success of the law, and the administration has struggled against political and operational obstacles to get the program running.

As the president spoke, Republicans in the House of Representatives renewed their plans to press for a delay to the healthcare program by attaching such language to the debt limit measure.

Obama had sharp language for the tactic, likening it to extortion and calling it "terrifying financial brinksmanship because of some ideological arguments that people are having."

Republicans are under pressure from some of their most conservative members to link government funding measures to cuts or delays to Obamacare.

A measure to link government funding for a year to a year's delay in Obamacare had been gaining traction in Congress. Last week House Republicans rejected a proposal put forward by House Speaker John Boehner and Majority Leader Eric Cantor as not strong enough.

Brendan Buck, a spokesman for Boehner, said Republicans are not threatening a debt default.

"The president only uses these scare tactics to avoid having to show the courage needed to deal with our debt crisis. Every major deficit deal in the last 30 years has been tied to a debt limit increase, and this time should be no different," Buck said.

(Editing by Philip Barbara and Vicki Allen)